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This is a very late addition to this thread, but I thought I'd put in my 2 cents worth (American) just for fun.
I recently swapped the entire internals from an FS engine (4cyl) 626 into my MX-3 gearbox. This got me a 4.11 final drive, different gear spacing, and a 0.717 5th gear. I now have 24mph/1000rpm in 5th - a huge difference. It's probably too tall a gear set for the K8, it just doesn't have enough low and mid-range torque. It is great for a KLxx 2.5 though. The reason the FS 4-cylinder would pull these gears is that it has its torque at much lower rpm than the K8. By the way, Probe GTs, at least American models, have the exact same gearbox as the MX-3, including final drive and 5th gear.
Transferring just the gears, without the final drive can be done, but requires more disassembly of both gearboxes to get the 4.39 pinion onto the new gears.
HOWEVER, the 5th gear only swap is easy, especially if you have an impact wrench to remove the nuts on the shafts. DON'T try to do this by jamming the small outer gear. You WILL break it! Trust me, this is from experience! Just take everything off of the FS box and swap it into the MX-3 box. Yes, you will have a big gap between 4th and 5th, but you should get 22mph/1000rpm or thereabouts, depending on tire size.
One more penny's worth. American driving conditions are radically different from those in the UK. I have taken my unmodified K8 MX-3 on several 500 mile trips, running an average of 80mph. Yes, the car did great, loved the revs, felt great, etc. BUT it is wearing on the ears to have such a buzzy, 4200rpm trip. The taller gears are MUCH nicer for long trips like this. If you're into café racing, then leave the standard MX-3 gearbox alone. It has great gearing for winding roads, lots of starts and stops. I STILL loved it after I put in the 2.5; 5th felt about like 3rd with the K8! But, the 2.5 engine is a torque motor and can use the longer gearing without giving up much, or any, acceleration.