The solid front fork machine would have to be 1980 to 1982 if it is indeed a 185/200. what size are the tires, 22 or 25"? That would help determine the model better. As for the engine if its a 185 it can be bored 4mm over if its a 200 then 2mm over, however I wouldn't recommend going full over right off the bat. Just bore to the size it needs to clean it up, put a 200x piston in it (which if it's a 185 requires a 2mm overbore to make it the same bore as the 200 but it still has plenty of cylinder thickness at that point), put a 1986-2002 xr200 cam in it an a genuine 200x carb. All that should wake it up quite a bit.
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