Been there myself. The lower engine mounting bolts are different sizes, one being 2mm shorter, so that it doesn't punch a hole through to the oil gallery. I had to strip the engine apart and get it welded. Splitting the crankcases isn't such a bastard as you might imagine. The barrels, head etc can remain in place, just remove the lower crankcase with the engine upside down. Of course, the clutch, oil pump and gear shifting mechanism has all got to come off. It's a good few hours work, it all depends on your skill level or confidence, don't forget the manual and plenty of hob nobs.
Let us know how you get on.