If the gear on the new starter motor has the same pitch then it will be a physically smaller gear. It will need to sit slightly closer to the flywheel ring gear for it to mesh properly. The bolt holes on the new motor body may allow for correct alignment so worth checking. (although I think this is unlikely)
If the gear is physically the same size as the old one the pitch will be different and may start to munch things pretty quickly if you use it.
Very difficult to tell without seeing it. (if you need some instructions on posting photos
http://www.auszookers.com/forum/viewtopic.php?f=1&t=50517)