I recently visited EDGE Performance near Oslo, Norway. They take Rotax engines and parts and replace components with their own higher quality versions to create new engines. For example they use the 915 pistons, which they said are excellent, in their 912Sti, and have developed their own ECU. They also make their own intake manifold and exhaust system. Their US dealer is STOL Creek: https://stolcreek.com/

I was very impressed with their shop, they have a dyno setup complete with altitude and environmental testing. Their assembly area was immaculate, similar to the Rotax line itself. The owner is an engineer and their head A&P is from the US, and said he knows Sebastian. They have an old 601 in their hangar they are refurbing and putting a Rotax on. They have a brand new building at the airport in Notodden (about 1.5 hrs west of Oslo).

I am looking at their 912Sti which makes 155hp, and has a selectable eco/power mode (they have their own ECU). 

If anyone is considering their engine but concerned about reliability and support, I don't know of any 3rd party manufacturer (using someone else's block) that is as high quality as theirs. I believe they also sell engines to the military for drones.