The meanest engine ever

I want to turn a PK50XL engine into a really fast one. What is the meanest I can go? Cylinder, crank, carb, gear - anyone have a proposal (or part list)?

 

Very unspecific question.

Anything goes.

Please tell us only how much money you want to spend.

 

 

Willing to spend, no specific budget. I have been thinking of a 136cc cylinder, but uncertain what's goes with this - what's the best setup (gear, carb, etc).