Rear Sprockets: Late model Husky WR125 & 250 rear sprockets will fit a GasGas. AFAM p/n 15206H, Sprocket Specialist, Talon p/n#TR124. Use a Talon (OEM) countershaft, it has the proper offset, that way your chain will not rub the cases.
Air Filter: UNI p/n#NU4112ST Honda CR125/500 1989-2001/CR250 1988-2001 KX 125/250 1990-91 & 1994-2001, KTM 250/300/360 1990-1997
Air Filter Twin Air KX125/250 1994-2001...I forgot the p/n...
Plastic: Rear Fender, side panels 1994-1996 KX250
Give Jim a call at SMS, he is the GG guru
Hope that helps...
Rick