The "28" would use a prop in the 18-6 to 18-8 range.

I have some of them. The converted ones have the C-H ignition. I thought it looked cool. They run perfect.

The original magneto flywheel ignition is cheaper, weighs more

Battery ignition gives just a little more power, starts easier, weighs less, and costs a lot more. The battery powered ignition makes the conversion go easier if you buy from C-H. They have all the other parts for your conversion also.

Enjoy,

Jim