I look at that thing and see a mojo monster waiting to get unleashed. I've had a couple of vintage Musicmasters and there's something about them. Not the finest construction money can buy, but vibe. For. Days!!!
I could have a new pickguard (WD), Strat shape bass pickup (Aero/Novak/Lace Sensor etc) & wiring ordered up in 10 minutes. If the tuners are shot, you should be able to shoehorn in some Hipshot Ultra-Lites.
What can't be easily sourced is the vintage, vibed-in, dried-out wood. The crude construction, genuine wear & subtle 1970s excess are all part of the charm IME. That's definitely worth some premium. $900 might be a little steep for how badly the last owner mangled things. But these are inflationary times. And 1972 builds don't exactly grow on trees.
My biggest concern would be the truss rod. If that is seriously compromised, that may be a cost-prohibitive fix at best. (Maybe that got spared from all the butchering... maybe not.)