You probably won't find that Manitou fork. There was a recall, because they (apparently) broke easily.

https://www.cpsc.gov/Recalls/1996/CP...n-Fork-Recall/

Also, the interior elastomers seem prone to decomposing and parts were hard (now impossible) to find:
Part hunting for '96 Answer Manitou Mach 5 SX HELP!!- Mtbr.com

To answer your question about changing the head tube, anything is possible. Go to a frame builder. Tell him the size of the head tube you want, and the material you want. Unless the original frame is a very good frame that the builder can verify materials on, he'll probably pass. If the material is actually weldable, you may find one who'll do it, but I'm guessing it will be a few hundred dollars. Then you'll want to have the head tube (or the whole frame) repainted. Another 100-200 bucks? And you'll end up with a frame that is probably inferior to something you can get off ebay for 50 bucks.

Your enthusiasm for this frame is clear. Also clear is the dream of a bike that is (in your mind) "ideal". We've all been there. Step back from your enthusiasm, figure out what your ideal bike would be, and investigate all avenues (not just rejuvenating this frame) to get there. Unless you are very lucky, and very patient, and willing to spend more on the refurb than you would buying the whole bike on ebay, you may be better off finding a whole bike, or at least a frame that has all of the parts the size you want, than you would be to refurb this frame by resizing head tubes.