OS 61FS as in 4 stroke? No way, not on a 60 size. It'll fly, but it'll be a slug. 91 at least. if you're talking older 2 stroke, then yeah if it's light. Forgive me I forget the nomenclature of the older engines.

If that's all you want to do with it, then it's ok if you just gotta have one. You'll have to fly extremely fast in knife edge, or it'll drop like a rock. With all those modifications you're talking about, you wouldn't really have a Kaos anymore. So why mess with it?

But that's just my opinion, the straight answer is it'll be ok if that's really what you want.

-Mike