I dont think it should be that hard, ally bar and cut a thread same size as the spark plug, wind in and weld, job done, drill new hole in the center and tap a new thread in.

BUT like I said I know nothing about the NSR motor as ive never worked on one.

To add to the head thread how many of you have eather O'ringed the head or O'ringed the top of the barrels? Its just ive not pulled one of these motors apart so dont know what clearences there are.

I gess 2smoke your going to get the Squish set at under 1mm or closer to 0.8mm. Should be easy enough with single heads. One big mistake though that alot of people make is skiming the head to do the squish and forgeting to recut the chamber to get the compresion back to near where it was.