Had the top bolt on mine start to strip out when I did the starter last year. Here's what I did:
1.Remove air cleaner, battery and tray, anything else in the way. This bolt is easier to remove from the top.
2.Loosen dip-stick tube and push it out of the way.
3.Use a very small screwdriver and make sure there is nothing in the allen hole of the bolt.
4.Take a good hardened 10 mm allen wrench and hacksaw off a piece about 1 1/2 to 2 inches long. Do a nice square job or dress it up on a grinder. You are going to want those nice, sharp edges on the cut off end.
5. If you can get to it, bang on the side of the bolt head with a hammer or such. May help break it loose.
6. Hold a long handle 10mm wrench in front of the bolt head and slide thecut off allen piece thru the wrench and into the bolt. Do whatever you have to to force it ALL the way into the bolt and make sure it is straight in. If need be, you can slide a pipe over the end of the wrench for more leverage and perhaps use a prybar to push against the end of the allen to keep it in the bolt.
7. now you should be able to pull up on the wrench and with luck the sharp edges of the cut off piece will hold against the remaining good metal of the bolt. just remember to keep the wrench tight up against the bolt head. This really helps to keep the allen from tilting and rounding.
8. Good luck and let us know how it turns out.