I bought a used Bullet which needed a top end rebuild.
I did that because I like to have an excuse to spend money to get some go.
Call me cheap if you wish.
From a bang for the buck point of view, just change the exhaust system and keep the original carb.
Take the money you saved on the carb and get the alloy 535cc and 8.5:1 piston.
Lap the stock head to the new alloy barrel.
Then change the secondary drive sprocket one or two teeth up.
Now you got an engine with more torque and power.
You will have a very tractable cycle with less shifting required in town.
If you want to change the air filter for a less restrictive one, put a 3 or 4 inch radiator tube between new filter and carb.
Rejet as needed.
You may like the result, and it'll come in under $1000 with relatively little work involved.
Stay away from tuning for top power and speed because you will have to make it turn faster.
Higher RPM are expensive in the long run and mean a lot more time in the garage.
Big end goes? Now you have an excuse
Peter