I am only putting the stage 1 mods on my kart, hopefully taking it nearer 10/than 6.5hp.
Hopefully the parts ordered, carb, exhaust, valve springs and timing key will take it up a bit.
I agree that to take it beyond this limit would get expensive and probably not add to the fun much.
It would be a shame to be restricted to a bog standard engine as I think the French are, part of the fun is tuning the engine and getting the kart to go as fast as possible.