From a stock kit
Plug and wire. I would rate about the best for the cost
Match exhaust and intake ports. Free it doesn't get much cheaper then that.
Air filter. Stock gets oily and dirty so quickly you are bound to lose power.
tuned pipe. A bit more costly then the previous, but about the best performance gain you get from these motors for time/effort/cost.
Shift kit. I would say is the best, more expensive then the motor. Being able to stick it to the stock chain tensioner "priceless".