6a11 1993 1.8L v6 galant, Buggered Auto trans

Hey guys.

My galants gearbox is buggered... in park, i cant rev over 2000RPM, same thing in reverse(cant move in reverse either)
Then in nuetral i can drive forward.. same as drive, 2, and Low, and overdrive does not work.

So im looking at getting a new gearbox. the trans code is f4a22, Now im from new zealand, so i guess its different to american models? but if any of you do know, could an auto box from a 6a12 be fitted, and work perfectly? Can i use any other boxxes? like f4a23?

Also how hard is it to put a new gearbox in? I have a little bit of mechanical knowledge.

Thanks guys