I agree that swapping the freehub should be pretty easy. I put a 4500-series Tiagra freehub on a Mountain LX rear hub for an '89 RockHopper, and it was pretty easy. As I recall, you might need to redish the wheel and use the cone and spacers from the donor hub.

Honestly, I would build a new wheel, but that's mostly because I like building wheels.

BTW, I have a front hub that matches your rear hub. It's been sitting on a shelf in my garage for years because I'm OCD about matching wheels and I haven't found the M750 rear at a decent price. If you would like to have it to pair with your, we can work out a trade. Alternatively, if you don't build a wheel with yours I'd be interested in acquiring whatever you don't use of it.