I did the same thing you did (follow instructions; strip the casing threads) and then did basically what Al described. Here are some specifics that may help you...
I did some research and determined that 1/4 inch by 24 threads per inch by 4 inch long bolts would be ideal for me. (Note that when you drill and tap your own holes, you don't have to stay metric.) I went to Ace Hardware and found a #17 drill. But I think that was matched to a 1/4 inch by 20 threads per inch bolt. So I bought a 1/4 inch by 20 threads per inch taper tap and handle. I think that is what I finally ended up using.
The bolts I used were Grade 8 bolts. If they are good enough for a high-perf Chevy big-block, they are good enough for me.
The 4 inch bolts are not long enough to bottom out in the hole, so need to worry about drilling too far, just don't push the #17 drill through the bottom of the hole. On my engine, the drill and tap broke through the outside wall of the threaded hole, but that turned out to be a non-concern; there was still plenty of thread to grip the bolts. If you had access to a drill press, your holes will be more accurate.
You will have to ream out the holes in the head to at least 1/4 inch diameter, so a 1/4 inch or maybe a 5/16 inch drill will act as the reamer. Do the same with the lower and upper gaskets also after determining if necessary.
During drilling, I cleaned away the drill chips, ensuring none fell into the crankcase. That could be a bad thing. During assembly, I used just a drop of oil on the bolt threads. I know, some guys advocate blue or even red Locktite. If you do, you will not be able to go back later and easily retighten head bolts.
I used a steel flat washer under the head of each bolt, then tightened them down to about 9 ft-lbs (108 inch-lbs). (I used a gallon jug of water hanging from the end of a foot-long wrench as a means of measuring the applied torque. It works!) During running, the cylinder will try to expand in length when it gets hot. The bolts expand at half the rate. In the end, the gaskets will be compressed more while running, providing a better seal. Later, when the engine is cold, you can back-turn the bolts a quarter turn and retighten to a final torque. I did that once after the first tank of gas and have not done it since in over 900 miles.
I did put a little plumbers thread sealant from a tube around the base of the spark plug. It was leaking cylinder gases and oil until I put that stuff on.
Have not opened my engine since. Mind you, it would probably improve with custom intake, exhaust, etc. But someday when I have a second bike I can ride while the first is in pieces....
MikeJ