Whether a Continental, Lycoming, or a Rotax, your plane needs to have air leaks contained (especially in the tail, right behind the cabin, as well as pressurized air coming into cabin) if you are to keep whatever heat you have in. Especially a fabric covered plane, as the cold and heat goes right through the fabric.

Exhaust temps on a Rotax is the same as any other gas engine, right around 1400+ degrees. So what kind of a heat muff you have on your exhaust pipe is the biggest factor there.

And if the heat muff you make doesn't give you enough heat, then add another from the other side of the exhaust (something you can't do legally on a factory built plane). And if that still isn't enough, take the heat right off the header pipes and put extra springs or coils in the muffs to retain more heat (that alone can make a huge difference).

And if you want even more cabin heat, add a heater core (or two) to your 912 in addition to the heat muffs. Or if you live in the Yukon, put a school bus heater core under your panel . Yes, a 912 will run a bus heater core without a problem. It will be heavy, but you'll have lots of heat.

Also, make sure you fly with a 180 degree thermostat (or radiator shutters) so you can keep your 912 hot on those extra cool days. Now your plane is just like your car (actually better because you also have 2 heat muffs in addition to the heater core, which you can make as big or small as you want )

And if that still isn't enough heat for you because you live north of the Yukon, capture all the hot air from the back side of the engine radiator and duct that into the cabin too (that's all I used on my Avid Flyer and even without a sealed cabin it was pretty good. I didn't need gloves unless it was below 10 degrees OAT).

With all that said, remember, you can't do any of that to a Cessna, a Piper, etc. Or at least legally. Oh, and not every factory built airplane has good heat like your 172. Talk to a few J-3 pilots and I'm sure they will confirm...

A friend of mine did a few things that I suggested above to his Avid Flyer years ago and he ended up moving the heat vents up into the cabin because the heat literally melted his boots on a 10 degree day. He had so much heat he never bothered to seal everything up like he should have (I personally would work first at sealing air leaks as I feel that is the best thing to do to minimize your need for a high volume of heat).

Bottom line, you do everything mentioned above and you will have way more heat than that Cessna. And since it is a homebuilt, if you don't like your result you can just keep adding muffs, heater cores, fans, more cabin sealing, etc. So you'd only have yourself to blame if your cabin is still cold.