RE: os 91 fs tends to die after 20-30sec idling
You may have to lean your idle mixture. The easiest way to get the engine set up is to have it upright on a test stand. Then the settings will be very close to what you'll need for inverted running. Also, make sure the fuel tank's not too high. That can also lead to problematic running.
Inverted engines tend to be more problematic than side or upright engines. That's because the glow plug can act as a sump at low throttle settings and get flooded out by fuel. An on-board glow plug battery that comes on at 1/3-1/4 throttle usually cures the difficulty.