I own or have owned most of the 2-stroke .91s. Right now I have a Magnum .91 2 & 4-stroke, 2 SK .91s and a Tower .75.

Tower .75:$90 bucks. Tons of power at high rpm. Likes a small prop. Would not be my choice for a radial cowled plane. The GMS is made by the same company in China, but has a Dykes ring. Needs a break-in for that type of ring. The Tower is easy to hand start and tune if you are familiar with Chinese engines. Most people make adjustments too large.

Magnum .91 is over $100, but a great engine. The 2-stroke has a good bit more power than the 4-stroke, which is a clone of the OS. I used an Ultra Thrust tuned muffler on a couple of planes and turned 12,000 on an APC 14-6 on 10% fuel. Very easy starting. This would be my second choice.

SK .91: $100 bucks. This would be my choice for a radial cowl. Ported for torque and large props. Very hard material and needs a good break-in. A 15-6 is the smallest prop that will run well. Smaller ones will lean out. I love this engine for big props. Break it in per their instructions and it is a tractor on big props. Try a 14-6 and it will lean out and fry in the air. Check the SK thread in the Engine Forum. They (at least my two) are easy to hand start, idle well, have a good mid range and hold top end on a big prop well. Excellent carb for function, although mine had burrs on the diagonal cut in the rotating barrel that I had to smooth out with a small file. I estimate it was from a dull cutting tool.