You're approaching a flying hybrid too traditionally. I'm not talking about adding a slightly larger starter that can "help" out during takeoff on a Rotax or Lycoming. No, the combustion engine has to go completely. I'm talking about a brushless electric motor in the front, nothing else. A battery and a turbine APU in the tail running a genset. Just like in the airlines.
The battery in this case made smaller and less heavy as it's only designed to be an energy storage device and be able to propel for maybe 30mins on its own. And the APU wouldn't have to be designed to excruciating standards as if it fails - you still have the battery that can execute a safe landing.
That would be lighter than a combustion setup and have all the benefits of electric and fossil. And even if it's the same weight, it's still much more desirable as it eliminates all the crap you to deal with with reciprocating engines.
As a side, and I say this owning two ancient Lycoming GO-435's on my AC520 (and I love they way the sound), but the problem is the combustion engine and the people that make them. They are the most traditionalist bunch I have ever come across and they've set aviation back 50 years. Let's talk some examples:
1. Why isn't the starter and generator combined to save weight? Ah, dunno, it works, don't mess with it...
2. Why are there still carburettors when the rest of the world moved to fuel injection years ago? Ah, dunno, it works, don't mess with it...
3. Fadec? Ah, dunno, it works, don't mess with it...
4. Will disintegrate if not fed ancient and eco unfriendly Avgas? Ah, dunno, it works, don't mess with it...
5. Magnetos that break mid flight when the rest of the world has had electronic ignition that doesn't brake ever for 20 years? Ah, dunno, it works, don't mess with it...
6. Exhaust Turbo that run the generator instead so you're not siphoning power from the engine? Ah, dunno, it works, don't mess with it...
The list just goes on and on. They're dinosaurs made by people why have had it far too comfortable and have not had to innovate. This is the drawback with type certification - once you're on paper as the engine that a type cert aircraft can use, they have you over a barrel. They don't need to do anything after that except cash your checks.