Are exhaust systems interchangeable UL/ULS/IS?

All of the engines have their cylinder exhaust ports in the same location or so close that the flexibility of the exhaust pipes will compensate.
So, all exhausts will fit all engines.  Fitting the exhausts into your cowling is another matter.
If you are just buying the pieces/parts and adjusting to fit, you should have no issues. 
If you are buying an exhaust that is already welded to fit an RV and trying to install it in a Sonex, it may or may not work.
It depends on where each manufacturer located the extras within the cowling that are not directly associated with the engine. (Radiators, Oil Coolers, Oil tank, etc.
And, then, of course, there is the cowling itself.

Good to have confirmation that all the exhaust systems will fit all the engines.  I've been scouring the internet for pictures of RV-12 exhaust, and the universal Rotax system, and if they're not exactly the same, they're as close as they can be.  Both are identically high priced as well. 

Sonex has warmed up to the 912, and they do make some mounts.  They don't currently make one for the Onex though, so I had to make my own.  The Van's or Rotax system that's tucked under the engine is about as compact as I can imagine, and will work as well any.  My plan was to install the exhaust next, then work around it with everything else.  The cowl will be a nightmare either way. 

Thanks,

Rusty

Russell, this company makes exhaust systems for the Rotax that may work. They have excellent cutomer service.

https://www.cktengineering.com/aircraft-exhaust-systems/index.html

Thanks.  I saw links to CKT in my searching over the weekend, but I'll try to stick with systems I can get in the US.

Rusty

Perhaps some background will help your selection.  the basic design was done some 3 decades ago with the introduction of the engine.  The 912 was only 80 HP and had an exhaust to work with it.  When they introduced the 100HP they did not change anything to speak of however it was recognized that too much back pressure will cause some power losses.  The back pressure spec and where to check is found in the installation manual.  (rotax-owner or Flyrotax web sites) 

When they introduced the iS engines Rotax did a lot of work to improve the life of the design, it was reinforced at the joint nipples on the canister and the internal baffles were changed.  Again the critical part when your done is be sure that you did not cause too much back pressure in the system.  

Vans uses a standard Rotax can and has special pipes made to fit their RV12.  Its a good system for that aircraft.  If you have to fabricate then the kit may be a good way to go however you will have to test your welding skills on the stainless pipes.  (i recommend some pre and post heat even on tig for this) Me personally am a wiz at gas welding but not so much tig.  Normally I get someone who specializes in that stuff.  The type of stainless is shown in the installtion manual so you can select the right wire and heat and not mix materials.  Be aware that the sockets and ball joints need to have a bit of movement to absorb vibrations, use a bit of anti-seize on these.  the mount springs should be set to pull the correct tension wo watch where you mount the spring hooks for a good fit.  

When you do get what you want then always draw a bead of high temp silicone across the top of each spring and put a safety wire in the event one may break.  (in a tractor your safeer than pusher on this item) 

Cheers

PS, had a friend years ago made a Sonex with a Rotax 914, it was great.