Thanks for looking in, I will need some hand holding on this one.
The top half of the crankcase will have the valve shaft running in a reamed hole.
No problems on the ends. The air connection will be on a manifold bolted to the back end of the crankcase and the front will be sealed by the face of the nylon gear with a wavy washer for tension. You might check my logic on the next bit. The engine will be running as a "two stroke", this will mean that at any "TDC" two cylinders will be "firing " together in opposite banks for 180 degrees. This means that the air supply is always flowing and the only back pressure will be from the springs pushing the steel balls into the closed position. So, hopefully there will be little air leakage along the length of the valve shaft.
I have built up a small hoard of PEEK from looking on Ebay there are sometimes bar ends for sale, it does not seem to attract much attention. It is harder than PTFE, a bit like acrylic and machines nicely. I will experiment with it for the pistons, using PTFE as plugs at each end of the gudgeon pin (wrist pin).
I made up a conrod, just to see if I could! The screws are stainless M2 panheads. Pictures of my method will follow when I make the rest.