I have not seen anyone do a replacement gearbox here but I might be wrong on that.
Even if you do find another gearbox by a different manufacturer it's gona cost.
Also you will most likely have to do a lot of fab work. I say that because Honda uses Honda splines and they're not like other machines so your axels will most likely not fit. Then you have to fab new mounts.
This is not going to be an easy project in my opinion.
RPM makes a gearbox you can check them out here:
http://www.rpmgearboxes.com/As you can tell it's a totally different axel drive.
CO
EDIT: In my opinion I would go to fagbook, craiglist or other media to find a used gearbox. This would be the easiest and fastest solution.
CO