[...] They were never engineered to run at 75% power for extended periods [...]
I don't think that this is necessarily the case. The Suzuki engines, on which the AeroMomentum engines are based on, were always also used in Suzuki outboard boat motors, where they often see high loads at high rpm, similar to the profile in an aircraft: Some idling / low load, followed by full throttle to get on step and then slightly less throttle to quickly get to the fishing grounds.
Many of the airboat customers, to which AeroMomentum has been selling their engines for years, often use them in fishing operations, where they run almost full throttle for most of the day. Quite a few of their airboat customers accumulate around 1,000 hrs / year with minimal maintenance and under high load. I therefore think that airboats are a unique, valuable testbed for AeroMomentum.
[...] front crankshaft bearings are not sturdy enough to handle prop loads, heavy, complex plumbing for liquid cooling, reliability, etc, etc... [...]
The AeroMomentum is using a gearbox (just like the Rotax 912) which takes the loads of the propeller and which has also been used in airboats for many years. Heavy is, in my opinion, true and wrong at the same time. Compared to an Rotax 912 our AeroMomentum is a fat pig, but also has 17 more hp. Compared to a 115 hp Lycoming O-235, it is however pretty light.
I weighed all the components of our engine, including the engine mount, bolts, the radiator, hoses, oil, coolant, etc. but without prop, spinner and cowling. All in, I came out at 241.75 lbs. AeroMomentum believes that they can reduce the weight by 10 lbs, what would bring it down to just above 230 lbs.
According to wikipedia, the O-235 weighs 243 lbs, without anything and dry. I guess the wet installed weight, including the engine mount and baffled would be around 280 lbs.
I also weighed the components of a friend's Rotax 912 ULS. While the engine itself weighed only 131.6 lbs, all the stuff around it adds up. All in it came out at 194.69 lbs.
So, overall the AeroMomentum is around 40 lbs heavier than the 17 hp weaker Rotax, but (I guess) around the same amount lighter than an O-235.
As for the complexity, we only have to install one singe water cooler. That's it. No baffles, no oil cooler.