I brought this up a couple of weeks ago on another thread dealing with endurance in UAV's. It's been eating at the back of my brain since.
I would think that two brushless motors would be necessary. One to drive the prop, the other to act as starter motor/generator on the IC engine. I would shy away from mechanical clutches and such. Trying to make things small and light tends to eat away at reliablity and durability.
The first issue I would tackle is how to start a .049 diesel converted motor remotely.
Then find out what altitude that sucker maxes out at.
The reason I would choose a diesel is two-fold. Fuel economy and no glow plug to power-up on startup.
The basic airframe would be based on a soaring glider with a big variable-pitch prop on the front.
The flight profile I would choose would be an ascent with the diesel running to charge the NiCads or NiMH batteries and excess power going to the prop. Get up to altitude where the IC motor has difficulties and shut it down. Roll the prop RPM's back and crank up the pitch to start a powered extended glide. I like NiMH batteries because they can be fast-charged. Might have to have a controllable cooling vent to cool the batts while charging while keeping them warm during discharge.
Yeesh! I just started rambling on. Just might have to get an .049 motor.
I hope this thread goes on. There is alot of potential here.