Dusty,
Unfortunately, as Sonny says, the 1100 was not a very good or popular CC model. The frame and sheet metal are the same as any of the 1000 series, however, the motor drove the transmission by a belt, and the transmission was a Peerless, aluminum case so it was really light weight. The 1100 was marketed as an "entry level" machine and produced by IH to compete with Murray, Craftsman, etc., and others that had "garden tractors" but all were the Briggs/Belt drive setup. It was just a cost/point machine to get folks in the door at CC dealerships.
If yours was swapped to an Onan engine, it was swapped in by someone at some time. I wonder if they swapped in a standard CC, cast iron rear end too? It would be easy to tell, if yours has the small shifter with round knob, and the shift pattern is left/right, then it's the Peerless transaxle. If it has the IH style long shifter with the triangle shaped rubber knob, and the up/down shift pattern, then its been swapped to a standard IH cast iron transmission like a Cub has.
If yours is locked up, and it's the peerless transaxle, it would be easy to drop in the drivetrain from any IH built Cub Cadet, gear drive unit, but you'll need everything, including the clutch pedal, all the linkages, engine, transmission, tunnel cover, etc. Probably better off at that point just to build the donor tractor instead of the 1100. Sorry to be the bearer of bad news.
Bill