I'm 'tinkering' as well. Application will be into a SPAD. I know nothing about the plane you are building.

I used the front starter housing on the Ryobi for a mount. It measures 1.220" in fore/aft dimension. The top mount holes are 3.60" c/l and allows the engine to center port/starboard in a 110 mm pipe flange. The carb protudes past the 'mount' but for me I can drill some access holes for adjustment screws and will use the modified choke plate and a bike spoke (2-56 ) to choke from outside the fuse

I like the idea of moving the prop aft on the shaft, W8ye.

All I have seen and read about for prop hubs are Alum.
Why do you fellas not use either UHMW or VHMW for the prop hubs? What temperature does the output / prop shaft have on a gas engine? The material is tough and machinable and it floats!! I'm sure the Alum hubs would sink! Alum Spec. Gravity 2.64 UHMW .928-.940 Tensile strength @ yeild 2900 psi ; Flexural Modulus of Elasticity 100,000 + psi. If it is OK for engine mouints, why not for prop hubs?

mikenlapaz