Just in case the thought has passed through, don't bother

[Ty Marcucci]

 

[Randy Powell]

Yea, Paul Walker once had an OS 40FSR with FP40 guts. Seems to work pretty well while it lasted. The idea of combining different parts to come up with a new engine has been around awhile. Can be interesting.

[Andrew Hathaway]

More fun stuff...

Fox 35 piston/liner will fit in a Fox 36X case and run, but the rod is different and the liner is timed differently.  Heads should interchange also.   The reverse might also be true, Fox 36 set in 35 case.

The 4 bolt backplate Fox 19/25 piston/cylinders will not interchange.  The 25 liner sits too low in the 19 case.

The OS 25 FSR and 25 FP piston/cylinders are very close.  The FSR has a larger boost port but the case isn't cut to match it, so operation should be the same with a fp liner.  As a side interest, the 20FP cylinder/piston, wrist pin, and head, will all bolt in a 25 FSR case creating... a 20FSR.   

I'm sure there are others, but these are the ones I've found just mixing and matching pieces in the shop.