I had it rebored and imported all the bits from the states plus a hydrotec phase kit (head,reeds,jets,exhaust) I can confirm that US engine internals are exactly the same as UK market ones.
Cleaned up the block to match the the ports in the liners,and generally cleaned up any rough bits and screwed it all back together with a carb clean and new water and fuel pumps,thermostat etc..
Ran it in for five hours then a further five hours of sympathetic use and it now happily pulls a 19'' prop when before it would only manage a 17''
I haven't checked the compressions yet but it feels and sounds a lot better.
Its on a 5m humber destroyer rib which is quite heavy, the engine is on a jack plate with a four inch setback and the vent plate is about two inches up from the bottom of the hull.
The 17'' prop was a standard alloy yam one ,the 19'' is a basic looking stainless solas, no cupping or vent holes,both are new and unmarked.
Any advice for a better prop please and should I run it higher ??
water pressure is 15psi at WOT , flat out RPM is 5500.
I'm thinking 19'' laser 11 or is there something better.
Many thanks for all the previous advice too !
(I know it'll never be fast but its fun and seaworthy as hell)