I've been thinking of getting one of those, it looks like a fun little plane. The bolt is probably metric. If you're going to the trouble of finding new bolts, you might as well go SAE. It'll improve your chances of finding a replacement when one vibrates loose. You should be able to get to the blind nuts, and as others have suggested, a little heat should loosen them right up. Knock them out and reinstall similarly sized SAE bolts and blind nuts.