I have a first generation Nissa Micra and the gearbox is on the way out. It currently only has a 4 speed gearbox!
Are any other gearboxes compatible with the car (maybe a 5 speed) and how can I find out?
For sure this kind of transformation is possible, but you need to take a lots of parts like the gear selector, gearbox stand, probably the two drive shafts(..) from a used Micra with the same engine. This need a good mechanic and the good used Micra, this will be not easy depending of your country or region, and expensive if you buy all parts somewhere on the internet. (Try Switzerland by phone for Japanese car..)
Changing gearbox by another is ok for technicals checks, but you cannot change the engine for a bigger one, it needs to stay the same, regarding to the registration document, and you have to stay with a manual gearbox.
So I know it's hard to hear, but this is probably the end for your tiny Micra, there are too many cars at so low price on the used market this will cost you probably much less.