Hello Barron,
I like your doodle.
I feel you will need an empennage as large and as far back as practical to have her fly well. In my opinion groplanes land at very low speeds and a large vertical stabilizer volume and rudder volume are helpful as the speed goes down.
A lot of horizontal stabilizer volume is helpful to manage pitch excursions.
A rotor mast in the proper place would also be good.
In my opinion the easiest thing to manage for your project is to bolt on a 160 or 180 horsepower Lycoming. Engine mounts and cowls are available and that means you don’t have to mange cooling or worrying about how you stress the airframe. You don’t have to mess with engine conversions, radiators and re-drives.
My 160 horsepower Lycoming weighs around 280 pounds ready to fly and actually makes 160 horsepower at 2,700 rpm. A parallel valve IO-360 makes 180 horsepower at 2,700 rpm and weighs about the same.
I feel your numbers are a little off on the Subaru engine. Any that I have flown spend a lot of time on the far side of 5,000 rpm and weigh well on the far side of 300lbs ready to fly.
Thank you, Vance