Hmm

Ok heres some possible solutions, assuming the lever mounting bit is not cracked. Sounds like the threads are stripped.

Just use a nut and bolt, wont look so hot but will secure the lever.

Another option is go with the next size up bolt but imperial not metric. Its usually a touch larger, and will probably tap itself. I would lube or grease it though. You will have to use an imperial allen key.

Stripping it could be caused by a few things, bolt not long enough, lack of lube on bolt. I remember A cinelli ALTER stem I owned came with bolts to short, and it was starting to strip I solved it by using bolts that went all the way through to the other side. Made it very secure.