You could A: drill out holes and re-tap for 1 size larger bolts. B: attempt to re-tap holes with original size and try new bolts. C: get a new balancer and bolts.
If it was me, I would try B first, it's the easiest, if it fails you haven't really lost much and your back where you started. Then I would try A which I'm sure if B doesn't work then A should solve the problem and then C if both A and B either don't work or your worried about those two options or just don't want to bother with them.
With options A and B, the new bolts should be grade 8, easily accessible at any hardware store and are cheap. If you opt for option C, remember to get a new crank bolt and thick washer as well, the bolt and washer are almost always sold together and are also cheap.
With the proper tools and know how (which it sounds like you have) should be a decent afternoon or Saturday job.