When it comes to engine upgrades, you will need to have 2 things in mind: 1) budget and 2) the extent of performance gain you want to gain. Then balance those two.
It appears that you are not adverse to spending some money so it will come down to how much "go" you want to buy.
Best big bore performance: Wiseco or Dynoman high compression kit; Mike Rieck stage 3 or 4 pit job with Kibblewhite oversize valves; 650 cam or Megacycle 126-20 cam; back-cut transmission; heavy duty cylinder studs; new primary and cam chains, cam chain guide and tensioner; CR26 carburetors; MotoGP 4-1 stainless exhaust.