RE: OS 120 4 stroke, 4 blade prop?
That is alot of prop for a lowly OS 120 surpass to swing. I had a 14X10 two blade on my 120 surpass and it was about right.
It can't hurt to try it on the bench and see what happens.
If you are looking for a engine to swing that prop, my guess is that you will end up with something more like a saito 180 or 220. Just a guess though...........