OS makes the 61 FX as well as the 91 FX. They are both reliable and a little more expensive than the competition. The 91 is slightly lighter than the 61. You will have to consider prop clearance. I have had experience with the 61 FX and it is a good running reliable engine.

That is a big step.....46 to a 91. Are you sure you want to go that high? What plane is it? The 91 will turn a 13 to 14 inch (33.3 to 35.8 cm) prop.

If you want a little more power without having to buy a new engine try a Nelson Ultrathrust muffler. Again, I said a LITTLE more power.