+1 been there, done that.Sorry to be the bearer of bad news but those bolts hold your crank case together around the crank shaft itself. And it sound like it's stripped. The upper case threads, probably not the bolt threads. The only way you're truly going to fix it is to split the cases and have an insert installed where the threads in the case should be. Don't know why this would be stripped in the first place unless the cases were split apart previously and over torqued on re-assembly.
Not fun but this is the case here.