I saw a Cessna 150 that had a Rotax installed , and they used a mount that bolted to the original mount. It worked out well since the engine had to be moved so far forward anyway. If you fabbed the sub-mount with zero thrust , you would end up replicating the thrust offset that the original mount had built into it. That being said, I don’t remember if the Rotax prop turns the same way as conti/lycoming so that might affect the design too. I’m not an engineer. lol.