I have a DLE 20 in mine and its more than enough power. Turning a 16-8 Master Airscrew prop at about 8,200 rpm or so. I got a Biela 16-8 four blade prop at a garage sale for next to nothing. I got a new plastic spinner and cut it for a 4 blade. Looks really great for static display. I ran the engine with it thinking I had so much power I could loose some and it would still be ok. Didn't work. Only turned about 4,400 rpm. Max power out put for this engine I think is at about 8,000-8,500 fpm. Didn't dare try to fly it. Now that I think about it, it had twice the blades of the 16-8 two blade and half the rpm. A DLE 20 will turn a Master Airscrew 16-8 three blade about 7,200 rpm and is plenty of power for a 65" to 71" wingspan warbird 11 to 12.5 lbs. A 14-7 four blade might work with a DLE 20, don't know, but it would look too small. Also the DLE 20 puts out more power than an OS 120 FS so its just not going to work for you 91FS. Now if you went to a 26 RCGF, or something like that it might? work, maybe a 15-7 which I think Biela makes. But I had to put both batteries back just in front of the tail wheel to balance with the DLE 20 and a 26 would be heavier. My RCGF turns the same 16-8 three blade 8,250 rpm. About 1,000 rpm more than the 20. Maybe I'll put the 16-8 4 blade on the 26 just to see what it will do. Also a gas engine over proped and loaded down will run hotter and require a much richer mixture. Ask me how I know.