Piston Replacement: Before ordering a new piston and/or rings after you've worn it out or soft seized, be sure you get the right size! Meaning, that the cylinder may have already be been bored once. The piston should have some sort of markings on the top of it if it's oversized. Ususally, .25mm, .50, .75 so be sure to clean the carbon off the top without removing aluminum. The best way is to measure the bore is with an inside micrometer or use a telescopic gauge & calipers to compare it to the manual/specifications.
Once you get the right piston for the bore, be sure to hone the cylinder to aid is seating the new rings.
Then you need to check to be sure that you have adaquate clearance between the piston and bore. Every motor is different, and different alloys can have different clearance requirements (forged, stock, etc.) due to their particular rate of expansion. 2-strokes can be different than 4-strokes. Generally speaking, a stock piston on a two-stroke should have at least .002 and up to .003" of clearance. Too much will make it rattle - too little and it will seize. Check with the manual or use the manufactuer's specs if using a custom alloy piston.
Also check the ring (s) for the proper end gap and side gap. The end gap is determined by carefully inserting the rings in the bore (no piston) and slip a feeler gauge in the gap to be sure it has at least .005 to .010". The side gap is where you slip the edge of the rings in their appropriate groove and with a feeler gauge check to be sure there is at least .002". Again, these sizes are all dependent on the specs of the bike or custom piston mfrs. recommendations.
Also, reassemble the top end changing the piston pin bearing if it's caged and simple. If it's a brass bushing, you'll need to carefully remove it w/o bending or beating on the con rod, press in a new one and then ream it to final size for a lubed pussy fit.