I have 6hp It is not really feasible to up rate it to 8hp because the manifold and reed valves are different and more importantly the crankcase is different. I guess the porting is different.
Ignition is different too
i dont know the year, but both motors are painted whait
both reed valves plates looks the same, crank case is also the same, ignition is the same( soon i will instal ignition taken from bad 6 hp powerhead to 9.9 74' -except ignition cam)