Hi reed,As the prior posters have said, perhaps if there are no cracks, you can clean up the threads by re-tapping..... but metric taps are not cheap (in fact, good taps of any system size are not cheap).
Helicoil might be OK, these plugs are torqued to only about 24 ft lbs. But, helicoil setups are not cheap either.
If you need to buy another crankcase bottom plate, cost isn't bad, as someone has already mentioned. But, you would have to buy the gasket so that it seals good upon re-installation. Going this route, you would accomplish 2 things....... you could clean up the bottom exterior and the bottom interior (including the oil pump screen, which is supposed to be done periodically anyway).
I think I would opt for this last option. But then, I have a massive motorcycle salvage yard near me........ probably 5,000 motorcycles in there..... most are Japanese.

~ ~ ~ jaknight ~ ~ ~