You're talking to one of the biggest Ford Escort nerds right here.
The MTX75 is out of the Contour and Focus. It will not bolt up to the 1.8 and it will not mount in the Escort chassis without a lot of custom work.
The G5M transmission does fine. The gears will hold up, but the shift forks don't like abuse. If your drivers can be gentle with the shifter, the forks won't bend and it'll last a long time. If you have a lot of arrive-and-drive people on your team, don't expect them to actually treat it nicely; in that situation, you'll need to change forks about every 5 races.
The differential and gear sets hold up just fine. What kills the diff is hard launches. So unless you're going to autocross or drag race the car, you won't have problems. They're durable under road race use.
I don't know if you can swap bell housings, but I'd say it's likely. However, this requires that you completely disassemble the transmission and move the case half and internals to the other bell housing. Not worth the trouble.
The internals are similar, but I wouldn't bet on being able to use ZX2 forks in an Escort GT. You could try it, but the fork diameters might be different.
Your best bet, it sounds like from the Mazda experts above, is to use a transmission out of another Mazda, like the 626. I can't personally confirm whether they work. In my mind, the transmission from a four-cylinder MX-3 is more likely to bolt in. The 626 used an F-series Mazda engine, the 323 used a 1.6-liter B-series engine that's near identical to your 1.8.
What OnkelUdo said about "anything from other Escorts will fit except anything bolted to the 1.9" means all chassis components. Sway bars, brakes, subframes, steering. Since you have an Escort GT, you already have the best components--there are no junkyard upgrades unless you luck upon aftermarket stuff. Be aware that Escorts with 13-inch wheels had smaller front brakes--those calipers won't work. Escort GT and first-gen Mazda Miata will be your only source for used rear calipers.
You can upgrade to aftermarket ZX2 springs if you use ZX2 strut hats. The Eibach and H&R springs made for the ZX2 are the best choices, and you can usually find a used set for cheap on TeamZX2.com. If you want to go super-cheaty, Koni strut inserts for a Mazda MX-3 will fit. Swapping in front lower control arms from a Mazda MX-3 will add a half-inch of track width to the car with no other modifications. You'll need to drill a hole in the MX-3 control arm to slip the sway bar link through, but otherwise it bolts up.