Take the top end off and send the jug to a machine shop and see how much over boring it needs, then buy the corresponding piston. Wiseco pistons are great just don't get the 12:1 buy a timing chain and guides and you'll be good to go also the head should be fine but have the machine shop check that it's flat and also have them lap the valves.
1985 ATC125M: Parts
1985 ATC200S: 84 200S Bottem end, 200x Head, 200x Carb, S&S Cycle Exhaust, Wiseco 10.25:1 Piston, Reground Cam with About 1mm more lift(unknown specs).
1988 TRX250X
1983 ATC200E: Total Pile.
"Bout Tree Fiddy" Master of Thy 22RE