Just catching up on the build. That crack is a massive bummer.
Personally, I'd go for the new cases. Although, a decent TIG welder could fix it. The amount of work is about the same -- you'll have to gut the motor in both cases.
If you're worried about the SN matching, take the old cases with the SN, and the new cases with a bill of sale to the State Police or DMV, and have them update the records. There should be some kind of procedure in place for that -- it's not uncommon.