I've flown the OS 40 FP on many planes over the years and I completely agree with Ken Erickson that an 11x4 prop will work best for you on that plane. I am an instructor in our club and I use a 5 lb Sig Kadet Mk II with that engine/prop combination flying off a rough grass field and it is fine.

I once flew a Jemco Mustang on an OS 40 FP. After some experimentation I found that a 10x5 prop was best for that plane. But that plane was smaller, lighter and faster, so the plane benefited from a little higher speed potential and didn't need the larger diameter. Your trainer is bigger, slower and heavier, so you want more diameter and less pitch. It will accelerate faster on your short field and produce higher thrust at low speed, so your climb out will be safer.

You don't need a new engine, you just need the right prop.

Jim