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The mc-52 isn't listed on roketapartsdept.com, but maybe one of the other 250s has the same part.

mc-54 250
mc-54b 250
mc-62 250
mc68a 250
mc-79 250

Here's the page to select parts by model.

www.roketapartsdept.com/catalog/Scooter_Parts-52-1.html

valves not sealing can be caused by the following:

1: the exhaust valve operates in a harsh environment due to High temperatures and corrosive fumes blowing across the sealing area of the valve and seat.
A lean mixture raises exhaust gas temps. making it drastically worse.
Long full throttle runs against a load will also push exhaust valve and seat temperatures up.
( both together will give you a you a non-sealing burned exhaust valve and seat in a hurry...)
The intake valve holds up far better due to the intake air helping to cooling it down.
Higher cylinder pressures increase corrosive NOX in the exhaust gases, temperatures and power.
( higher compression ratios and high octane gas at high rpm can add enough "WHEEE!!" factor from the additional power to make the shorter parts life a justifiable expense for some...)

2: worn valve guides allowing the valve head to rock back and forth slightly against the valve seat in the head every time the rocker arm pushes down on the valve stem. ( there's a small side force present also, normally it's taken up in the rocker arm sliding across the valve stem end. ) This can wear the seat oval shaped or cause the valve to wear deeper into the seat on opposite sides leaving the rest of the seat higher which then holds the valve off the worn areas after the rocker arm pressure is fully released.

3: lead in gasoline helps lubricate exhaust valve and seat reducing wear.
Installing hardened steel valve seats is required to run unleaded gas and not wear down the exhaust valve seat too rapidly.
Leaded gas is gone due to EPA requirements.
Adding the lead back with a leaded fuel additive will give longer valve life and create toxic lead pollution...

4. Exhaust valve depends on seating fully on the valve seat over time for cooling.
High rpm causes valve float and the valve may also bounce back open on closing,
both raise valve temp due to reduced time on seat,which can lead to a burned valve and seat.
( fixable with a stiffer valve spring, which will also allow engine to operate at a higher rpm,
but this is a compromise, it gives higher wear on cam lobes, lifters, rocker arms and valve stem tip. still worthwhile for some for more "WHEE!!!" factor...)

5. non-hydraulic lifter engine with too little valve lash clearance. if it even runs, the valves are not getting cooled enough by being fully seated.

Personally, I would say the worst factor leading to burned non-sealing exhaust valves is the factory jetting being slightly too lean at full throttle.
Next would be running a little engine for an extended time totally maxed out when a larger engine running nowhere near maxed out would be a better choice.
( both conditions together will give a burned exhaust valve quickly...)

When using an impact wrench, always use a 6 point impact socket. Regular sockets will split, 12 point sockets will round off the nut or bolt if it's really on tight.
Sometimes I'll use an inch size socket or wrench on a metric fastener ( or vice versa ) if there's one that fits tighter. 13mm is close to 1/2 inch. 14mm is close to 9/16. and there are others.
To get off a rounded off nut, sometimes you can get by by grinding off the entry radius on a 6 point socket, then tapping it fully down with a hammer. Usually it's just the outer half that rounds off due to a loose fitting large entry radius 12 point socket or wrench.
One way I've used to get off rounded off nuts is to grind opposing sides until just barely into the tips of the threads all the way across the nut, then take a chisel to the thicker part of the nut and beat on it in the direction that unscrews it, the nut will spread apart enough to unscrew if it doesn't split in two.

On the 8 pole flywheel I have that's now only good as a paperweight due to the center being trashed, I measured: 90mm ID and 111mm OD.
OD is slightly bell shaped. 110.75mm at outer edge when installed , 111.05mm at inner edge when installed.

If the engine hasn't been abused and they're in spec, there's no reason not to reuse rings.
Given an In spec ring that performed for many hours flawlessly, vs a made in china untested new ring ,
I'de reuse the old one..
In theory, If the engine has overheated in the past, the rings may be low on ring to cylinder wall pressure and cause low compression and allow an excessive amount of oil to get by.
I've heard others say rings loose tension after running a few hundred hours even if not abused,
then blowby causes build up of deposits which causes rings ri stick on piston.
My experience has been: on engines with the rings stuck on the piston due cooked oil caused by piston high temps, ( I've only seen this on engines that have been run way too hot for too long.) after freeing up the rings and breaking the glaze on the cylinders they've all ran fine afterwards.
( sometimes gotta deal with one or more of: piston so loose in the cylinder it rattles around, scored cylinder/piston/rings, oval shaped valve seats due to worn valve guides, burned exhaust valves/seats )

disconnecting the vaccum hose for the carburetor air cut off valve ( de-accel valve ) will cause more popping on de-accel.
Don't forget to cap off the disconnected vacuum hose and air valve fitting.
When the throttle is slammed shut at high rpm, the mixture goes lean, occasionally too lean to fire, the unburned gas & air accumulates in the muffler until the extreme high vaccum goes away as the rpm decrease and the mixture starts firing in the cylinder again. The unburned gas being dumped in the muffler can be set off the red hot header pipe or cat converter ( initually ) or later by hot exhaust gasses when it starts firing again when the high vacuum goes away as the rpm drops off.
Later carbureted cars used to have a dashpot to limit how fast the throttle closed from just off idle to idle to reduce exhaust popping and for emissions.