Hello Mikew!
Are you sure that the lack of power is coming from the engine ?
Maybe interesting to check the engine with compressed air. Check if there is a leak out of the engine (gasket?) , I doubt it.
If you have a rev's counter , check if you can reach 2500rpm (or more with 2 b (30 psi) pressure. Not in charge.
Do not forget to flood the engine with light oil for the breaking. Check if any stiff point.
Those engines like oil as much as an 'Aussie' likes beer!
[size=78%] [/size][/size]Let us know. You will make it.[size=78%][/size]Regards.[size=78%]