Any engine will turn a three-bladed prop. Just drop 2" off the diameter of the recommended two-blade and keep the same pitch.

It is not a direct cc to inch conversion for what a model can fly when hopping from glow to gas. .91ci converts to 14.9cc and 1.2ci converts to 19.66cc. But I know folks who insist a 40cc or even 50cc is needed for a 1.2 2-stroke glow size model; which is insane but so are they. Gas engines have "baggage" of either a magneto or a flight battery for EI that glow gets away without so you need to "boost" the gas size in the swap . . . and usually reinforce the landing gear also. A model that calls for a 1.8c.i. glow will probably enjoy a 30cc or larger gas engine. I fly a 43cc Fuji EI in a model designated for a 1.8ci glow and it is a good marriage. For your 1.2ci example a 26cc should be good, 32cc for some real power.