do you have a timing light?
if so, start it up, and see what timing it is running now.
then take the stator off, slot the stator mounting holes 2mm so it advances the timing a couple more degrees.
refit it with the stator turned towards the way you filed the slots
then try it out, if it pings under load then stop, turn the bike off, loosen the stator bolts and move it back to where it was originally, then lock it up again
just looking at the ZS 250 stator, it bolts to the inside of the ignition cover
so if you take the cover off the engine it should be easy to undo those 3 bolts and remove the stator
so those 3x holes need to be filed 2mm longer on one side to advance the timing
if you mark a circle level with the insides of the bolts holes, then file/slot them and follow the line, so the slots will be slightly curved
hope you understand what i mean ?
so before you start, take the chrome plug off the top of the stator cover, and hook up your timing light to the battery and coil lead
start up the bike, and shine the timing light into the hole, you should see the flywheel marking lined up at 12o'clock
now raise the idle speed up to about 2000rpm, the marking will move to the left a little as it advances
so when you file the holes out on the stator you will be moving that timing mark to the left a touch more than where it sat at idle before
2mm is only going to advance it 5 degrees maximum, the engine wont mind that.
i run my Cleveland almost 12 degrees more advanced than standard, just set my distributor to limit my timing mechanically.