Scott,

Make sure you get the prop hub that has the big bolt axially down the
center, not the one with two bolts that are cross-drilled through the
crankshaft.

As for improving the Onan installation, the single biggest improvement you
could make would be to do the cooling system properly. The Onan wants to be
cooled from the exhaust valve side which means that to do it right you'd
need to get the cold air to the back side of the engine and have it flow
forward. Just copy the stock Onan cooling shroud (in aluminum or glass) and
get the air flowing in the right direction.... You could have the cooling
outlet(s) on top of the cowl a la Catbird or Adam 500....

JMHO,

David J. Gall