So I have a 1996 X-90 (same engine as tracker, sidekick, vitara of that year). It started losing power on hills, then all the time, then started idling rough and bogging down when I give it gas, then I had to feather the throttle to get the rpms up enough to even drive it. Now it's pretty much undriveable. I changed the fuel filter and it didn't help. Any ideas?

When last did you do a "tune-up" - you know p[lugs, wires, etc.?
A compression test might be a good place to start - what do the wet & dry compression numbers look like?

When was the timing belt last changed? Was the big bolt removed to remove the entire gear, or just the small ones to remove the pulley?
There's a possibility the big bolt either came loose or wasn't tightened enough and now the keyway in the crank is 'boogered", along with the key and possibly the timing gear.
Or the belt skipped a tooth or 3.

There is a lot of things that can cause the problem you have , have a glogged cat can also do the same thing that you have going on