Airbox lid removal (requires rejeting)
Clear tube mod for carbs (I think this is a sticky)
For more top speed you can cut the inductor wheel back to 1/2 or only 2 teeth ( inside clutch cover and for max speed only)
If you want less top speed and more punch off the line put in a smaller front sprocket.
Full exhaust - not so cheap
+more that would be best if you just read
Aside from hacking away at the speed wheel or disabling the parking brake and reverse limiters or making sure the carbs are properly adjusted there is no such thing as a free 660 performance mod that I can think of.
As alluded to in your other thread, removing the baffle and opening the airbox, with the proper jetting, can add a very modest amount of performance - a small fraction of what a good aftermarket pipe/exhaust with the same airbox mods and proper jetting will do for you.