from what I recall, the K&B series 75 was a plain bearing version of the .35 that you can see in my C gas build up. We used to use them for a U control Sports racer model that we flew here in the midwest.
As far as performnace, they were not bad. The engine needed to have a good ring fit. This is critical to its performance. They definately do not rate as hi as the newer schnuerle engine, but are not bad. Also, because of the plain bearings, they have a bit more bottom end drag which slows them down a small amount.
I remember we ran 30% in it with no problems and probably would need a 10×4 prop for it.
All in alll, not a bad engine, just not the hot rods that a new schnuerle is…
The pic is of the ball bearing version of that engine (plus, it is a lapped piston as opposed to the dykes ring…)