SWAP UL cylinders for ULS

Cylinders and pistons has been done.  Not sure of what it involves maybe a call to some of the service center would help. 

Does work (cyl. + pistons). Expect 95 to 98 hp.

You would have to machine the 100hp cylinders to fit the 80hp heads and change the starter to an HD starter.  It's no easy task and not for the inexperienced. It would be much better to sell your 912UL and find a 912ULS instead.

You can also check out a few aftermarket performance companies such as BullyHawk Performance http://bullyhawk.com/# or Edge Performance https://www.edgeperformance.no/ If you are looking to modify your current engine, however, this would mean you no longer have a warranty or Rotax Tech support from service centers/dealers as its no longer a "Rotax" engine. 

You will be creating a FrankenTax engine.
Browse through the Parts Catalog and take note of the differences in the parts called out for the ULS and the UL versions.
Besides the obvious Pistons, Cylinders, and Heads,  a quick look also shows variations in the Gearbox, Carburetors, Block, and Camshaft.

Thankyou all for the advice. Sounds do-able but not the easiest way to go. I do have a second hand 912S with 1800hrs. Maybe the better option.