Fairy overdrives in good nick are rare as rocking horse sh1t. If you find one the selector rods are usually missing. The new GKN will fit a 5 speed (LT77) g/box, but not a 4 speed. At £850 + is it worth it?
4 or 5 speed wise, see if you can shove the gear lever further right than 3rd. Easier still is reverse top left or bottom right? I asume you use reverse.