If it won't move, but make a noise, your diagnosis is correct. If there's no noise from the transmission, check that (a) the dump valve (freewheel control) is NOT engaged, and (b) that the drive belt hasn't snapped/come off.
Unfortunately, a new transmission is ££££ - IF you can find one.
phil_saunders(a)bigfoot.com
Phil Saunders, July 2009