It all depends on the quality of the case used to make the rep. Unfortunately, with a few notable exceptions (MBW), there is little consistency within manufacturing runs of reps, so it is impossible to say, beforehand, whether a given rep will accept gen parts. In these cases, I think your best option is to follow Nanuq's advice. Measure your case & then see what fits. You might even get lucky & find that your case dimensions are close enough to the gen's so that you can install a gen crystal (which may cost more than your rep).
The crystal for a gen 1675 is the Tropic 116 (sorry, I do not have its dimensions).