I raced Rotax for 4 season, and Yamaha the past 2. Maintenance wise, it’s not straightforward.
Rotax: no problem whatsoever. Yamaha: two race days were lost as I trouble-shooted poor engine performance; eventually, I sent it to a guy with a dyno who figured out it was loose wire in the coil / magneto side of the motor ($90 repair).
Rotax: had to replace my starter once. Yamaha: all sorts of issues with a POS starter I bought used from a dealer. Bought a BRAND NEW mini-starter in September, and sometimes the thing won’t shut off!
Rotax: total BS; you need to spend megabucks to nail down weather conditions to achieve optimum performance; apparently, the new carb will solve the problem. Yamaha: generally worry-free, but you need to keep an eye on the screen, and the system needs to be rebuilt way more often
Rotax: the old one was awful; the new one a breeze. Yamaha: my dry clutch is complicated and needs constant attention; have broken one, and twice the gear screws have worked themselves loose, which locked up the crank
Rotax: I put 33 hours on mine and rebuilt the top end once ($300) and a complete rebuild once (about $900). Yamaha: bought used and crapped out after one race (bought it from the same guy who sold me the crappy starter); full rebuild was $700, raced less than 8 hours, then rebuilt again for another $650-700.
Rotax: clean every now and then. Yamaha: no power valve
Rotax: somewhat of a pain to mount (needed to change to a 10-degree motor mount, and even with 4 springs it leaked and made a mess). Yamaha: no real problems, other than accidentally leaving a paper towel in it (which ruined two race days as I tried to figure out why my motor wouldn’t run).