I have a Marin small steel frame MTB circa 1997. Is it possible to upgrade to today's parts such as 10 speed derailleurs and shifters. I'm pretty sure disk brakes are out of the question. If so any suggestions on where to start.
As others have mentioned you'd be cheaper getting a new bike than upgrading. The frame also won't support disks, although if you replaced the fork you could fit a front disk.
Importantly the geometry of a bike has changed quite a lot since those days. I've got a marin pine mountain from around that time and it feels really unstable compared to a modern hardtail and is not corrected for a suspension fork.
However to directly answer your question I think if you replace the entire drivetrain it should work. The rear hub spacing should be 135mm.