So I picked up a TT600 for cheap to resurrect my ill-fated XT600. I put a new CDI in the TT600 and it fired up great. I then put the TT600 engine in my XT600 frame, swapped the side case from an old XT600 to get the 12V stator and tried to fire it up. No spark. I'm pretty sure I wired it up correctly.

Do I also need to swap the rotor for some reason? If not, I will try to test the stator and re-check the wiring yet again. Other tips?

Yes, swap the rotors too. The system on the old TT with 6V is different from the XT's with the 12V system. Although the figures in this text refer to the charging / lighting system that has nothing to do with the ignition, I am just almost sure that the rotor of the XT should be matched with the stator of the XT, same for the TT's.

Rgds,
Auke

__________________
Auke
http://aukemarieke.blogspot.com

You were right.

The official answer is yes, you need to use the XT600 rotor with the XT600 stator. The TT600 stator is smaller in diameter (and lighter) compared to that from the XT600. Other models may be different (I've heard of heavier rotors on later years), but it held true for my 1984 TT600 engine and 1989 XT600 stator/rotor.

Fired up on first kick and thumps great with no strange noises. Maiden voyage tomorrow.