You need to work off three concepts. Higher compression, more gas, more air.
go buy a head with a larger compression ratio. (or find a 4hp head, Im not sure if the two match up, look into that)
To get more air you need to port the intake and exhaust. Nothing major right away, just smooth out rough edges and casting imperfections.
And with the increase in air you will need more fuel. Which will consist of drilling out/buying larger jets
I know i gave a vauge answer but either google the stuff i have said or wait for others to reply