Best option for power is porting and different pipes. Your current pipe is a low end pipe, buy CPI or Shearer pipes and have you engine ported and stock crank welded. Advance your timing to plus 4 is cheap and free.
For another 400 you could buy a 4 mil crank and increase your hp to 75hp with that set up.
VF4 reeds, higher compression, larger carbs (although not necessarily needed) will all give you gains.
Check the dyno section for 4 mil stocker engines and you will see the gains available