You might be able to use a tap to repair the threads. Use a little grease on it to keep the shaving from falling into the cylinder. If this doesn't work then you may have to drill it out or install helicoils into the heads. If that doesn't work then you need use a bigger size plug.
First you need to remove your head to insure you do not get metal shavings in cylinder. Then you need to find out what size hole and thread pitch it is. Then you need a thread chaser (buy at local auto parts store). Run the chaser through and reassemble. Done
The best way is to remove the head (so you don't get metal in the cylinder). Then purchase a Thread repair kit in the proper thread size, follow the instructions.
Repairing stripped threads on a spark plug is not recommended, always replace necessary parts for a motorcycle; however if you MUST repair, using #1 or #2 loc-tite is an option.