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I wouldn't hesitate to re-use those cases. Most cases look like that after a top-end failure, and I've never had a problem with using them again. For your stator screws, the best tool for getting those out is something like this: http://store.snapon.com/Impact-Drivers-3-8--Set-Impact-Driver-8-pcs--P6… Doesn't have to be a Snap-On of course, lol. If you take material out of the crank case...

Nice bike! My first real big bike was an 89 RM250...I will have to try to find a decent photo of it. It wasn't as nice as that one though.

I don't know about car mechanics, but I've seen some ugly work done by electricians getting their hands on a motorcycle or atv. It turns into Marrette overload!

You could try to grind some flats on what is left of that oil drain bolt and then grab it with vice-grips. You will have more room to work on the oil drain once the engine is removed from the frame.