Paul,
You can use OS 91 4-stroke, but be careful not to make the plane too heavy. Although it is lighter than 61FX w/ muffler (670g), 655g weight of the 91 is kinda too much compared to 520 grams of YS 63. The plane is designed for OS 70 (600g), 46FX 50SX (under 500g).

Also, make sure there is enough ground clearance for large 14" prop. You don't want to fly too fast.

The combination should be more than good enough for Sportman pattern. Although all of the District 8 pattern guys are flying 90 and larger size pattern planes, you can beat us with lots of practice!! Burn lots of fuel with a coach standing next to you as a caller/mentor!!
Hope to see you in person this season.