From the spares of my rally car but not really required as I'm working on a new box.
First up is a Hewland 5.0:1 straight cut final drive. I believe this was used in the old Focus challenge rally cars. Very rare item. Chris Birkbeck lists these for almost £1500. Should work in any MTX75 gearbox and is probably the cheapest way to make your car accelerate notably faster.
£300ono
Next up is a Quaife 4.6:1 helical cut final drive. These are almost £800 new. I don't know if they work with the standard gear kit, or if you need to be running a Quaife gear kit.
£200ono
Open to offers for both.