st70 upgrade

Hi,
I have a 6v st70 but would like to keep the engine a honda but with a bit more power.Can If upgrade to a 90cc piston and cylinder head would I have to change things in the gearbox aswell?

85cc (51mm) is what I would recommend the maxium.
It's best to change the oil pump as well.
And you might need to upgrade the clutch.