I have direct replacement piston kits at a good price for your CFI 800. I also have Fix-it kits available with the shim and longer piston. See link.
https://www.gobigparts.com/Snowmobile-Products/Engine/FIX-IT-KITS-PISTONS/
https://www.gobigparts.com/Snowmobile-Products/Engine/FIX-IT-KITS-PISTONS/