Mike, the needle valves on the OK engines are quite crude. There's not really much of a taper and as you mentioned just a rounded end. I have seen some examples where a Cox spraybar has been removed from a production backplate and JB weld was used to retain it in the case. The stock example works however it's very short placing your fingers in close proximity of the prop. Perfect made a replacement needle with a spring between the needle and the knob which in fact does utilize a taper that fits the OK spraybars if you can locate on it's a direct replacement.

Under magnification the end of the "needle" does have a blunt taper and reduces slightly in diameter, so it appears to be stock and not broken.

Now that that is resolved, what FUEL is this oldie going to require? Can it be run on modern CL fuel with 22% oil, half castor half synthetic? I have a can of ancient Cox fuel, unopened, that would probably work but not sure. Any help would be appreciated, I have never run an OK engine.