The Cat 2000 didn't didn't have a gearbox, do you mean transmission housing that holds the front and rear diff's?
I am not sure they made alloy ones, I bought an original plastic front transmission housing last week from Wheelspin models was only about £7. Rear transmission housing's seem to be harder to come by.
Have a look at this manual, you might find the part code your after.
http://www.competitionx.com/pdf/vehi...-ec-manual.pdf