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You have a few options. Depending on what frame repairs are needed, you might considder looking for an 87+ engine. These engines have  better aftermarket support so finding parts won't be as difficult. Plus you will have an engine with a 6 speed tranny instead of a 5 speed. The engine is a direct fit except that you will either have to put a wishbone in the down tube where the exhaust pipe bolts to the cylinder head or weld in two separate tubes going from the lower frame to the upper ones.

If you want to keep the original engine, you would be better off to do a complete rebuild with new piston & rings, seals, re-cut head, and bore & hone, not just bearing replacement.