I have a 1972 R75/5, 50,000 miles... I am rebuilding the engine.  Crank is out, just pressed new bearings in and noticed a small crack at the front of the case through the bearing holder 'hole' at around 7 o'clock, passing downward about an inch, through the internal webbing.  Looks like it has been there for a long time from the staining, and machine marks...maybe since 1972?   Since it looks like it has been there a while and the forces from the front bearing are distributed pretty broadly due to the fit of the bearing holder,  I am considering drilling the end of the crack and filling with appropriate epoxy.  However that seems like a half measure since I am this far into it......on Ebay there are used cases that are cheap enough, is there any reason that the bearing holder, cylinder, etc parts off my old case won't fit a different serial number case?  Any advice?