Gerry.
I am sure this engine is a Horner by looking at the shape of the cylinder heads in your pictures,He made (2) types of heads.It appears from looking at my brochures you have the later set which in the brochure he claims was a TOC version.In one add he said it was 9 to 1 compression and other he said 7 to 1, Weight 5 lbs in one and 4 lbs in other,RPM stated was 6500 to 11000,Plugs were NKG 10 MM.Your appears to be a converted ignition as the originals was a fixed type with pickup mounted on two lower bolts going thru engine crankcase and coil box was a rectangular aliuminum box with no markings,As far as props I would reccomend a 20" or 22",On my 4.8 conversions I use 22x8/14 or 24x10.The engine you have I would say is a 3.7 CI.Hope this helps. Bob