GX 160 is 5.5 HP, But if you play with it a little, your likely to get a bit more.
KTM is apparently 9HP, But at 10 000 RPM, With very little low down torque in comparison.
It's like the difference between an old V8 and a late model Turbo 4 cyl.
The Turbo might have more HP on paper, But you have to ring it's neck to get said HP.
I like old V8's, the way you can idle around in them, and tromp it in any gear, not have to shift down.
I've never done a 4 stroke build but have a GX160 here, however don't have the right frame to put it in.