FYI
http://www.lt-racing.com/html/xr250r_s.html
Les and I have been riding xr250s since 1987....
I would go with a J & E piston versus the wossner. Although a Honda oem 1985 xr250r piston 4th overbore is a good alternative ( if it is still available).
Your other option is to pick up a sleeve and piston 80mm from thumper racing to go to 300cc. The cases will need to be split and bored to accept the larger sleeve.
Best pipe back in the day was a white brothers magalloy.
The stock pipe with the baffle removed has good power...
For carb, I would stay away from from cheap kits that use 2 stroke carbs.
Stock cam works best for woods.
Jeff