I have a Superstar 60 ARF trainer.

This is roughly the same size and weight as the Right Flyer 60 you are considering.

I use an OS 65LA in it, and it has plenty of power. It will take off with 1/2 - 3/4 throttle.

Even at a higher elevation, a 60 size 2 stroke should be adequate.

You don't really want to run a prop larger than 12" diam for ground clearance.

Using a more powerful engine than you need, and increasing the pitch of the prop to load it down to the right RPM will make the plane fly faster than you would want for a trainer.

Stick with a decent 60 sized 2 stroke, there's plenty of other planes you can use it in later.....