How much can you afford?- it's a slippery slope. Go really big and you are re-using just the tin and some of the bolts. If other parts are still good then 87-88 mm P&C (1641/1679 cc) for under $300 (parts) or a 1914 cc (commonly called a 1915) would be $600 (base) but requires complete disassembly and machining of heads/case to 94 mm cylinders. Other combinations take a new crank followed by P&C, cam, heads, and carb (plus larger diameter heat exchangers).