You may care to start by doing a thorough Googoo search on all things Thielert.
AIUI, there were problems with very short-lifed reduction-gearboxes and ISTR a clutch problem as well.
Weight, vibration and , to a lesser extent, noise, are also issues. cold operations can also be difficult with Diesels, though Ibelieve the Thielert ran on Kero/ jet and not Derv. (so no waxing issues) The fact they went bump quite spectacularly, ,left a lot of temporarily grounded aircraft...Iassume the Lycosaurus was a modification, retro-replacement, which wouldn't have been cheap.
The Austro engine is an unknown quantity to me, It seems to be ticking along in the background, whereas there was a lot of razzmatazz surrounding the Thielert which, if it had been properly sorted, would surely have been a front runner today.
Rotax were virtually unknown 25 years ago....you had a choice , in a Flexwing, of a Robin, or a Rotax...(when do you hear of Robin today?)
Do a lot of research ,first.