I think the bolts only go into the sprocket, so you should be all set with new bolts. That plate has to turn, though, so if it's just the head that's broken off, there may be some stud left preventing it from turning.
As for the gas in the case, I don't think you would need to bore out the jugs just for that (unless you;re looking for an excuse, then go for it). You'll want to get that gas out of there soon, though. And get ready for a couple of oil changes - once for fresh oil, run it for a couple hundred miles, change it again.