Are you talking about a really old style F120? Chances are a modern 120 converted to gasoline, or a purpose built gasser fourstroke (say, a Saito FG21, or such) would allready achieve the same output hands down.

But for engines in equivalent state of tune, rule of thumb is that you need 25% more displacement on gasoline compared to glowfuel (so a 150 converted to glow should by and large equal a 120 running on glowfuel).

Hope that answers your question?