The ct110 engine has several key differences from an atc90, 110, or even a ct90. The architecture is much closer to that of the atc125 lay down engines. The cylinder head has two bolts on the timing chain side of the jug as opposed to one, the intake port bolt spacing and angle on the head is different, and the oil galley between the left crankcase half and stator cover is different. I built a hybrid ct110/atc110/S90 engine a few years ago, and these are all the differences I remember. There may have been more.
1985 Tri-Z 250
1985 ATC250R