Bolt,

Back in the 90s when I built my first CG Chipmunks, the first one flew on a K&B61 and the next one on an OS91FS
Both flew extremely well with lots of power to spare.

I had issues with the K&B where I seemed to have fuel draw problems but when I added a Perry pump, it seemed to fix it.

I agree with Bill, a FS120 is a bit over the top. You want your Chipmunk lightweight. These are amongst the best flying planes you can get your hands on and should be built light.

Ari