I have a 76 Puck Maxi 9W/160 miles on the OD). I bought it for $50 from a friend who was cleaning out his garage and moving. After cleaning out the carb., and draining the tank & cleaning out the rust in it. I had to replace the clogged gas petcock. It ran fine. It would do 30 on level ground but wouldn't pull me (230#) nor a female friend 120# ??) up some of the steep hills in my town. I invested in a 70 cc euro kit w/a biturbo (about $300). It runs great and will pull me up the steepest hills (w/a flying start of course). I ride it to the commuter train station 14 miles away at 35 to 40 mph w/the throttle at 3/4. Since I'm still trying to break it in, I don't run it at full throttle. Besides, what is the use, the speedo tops out at 40.
If speed is what your looking for, the cheapest thing you could do is change the front spocket from the stock 14 tooth to and 18 tooth one (you will lose a little on the pick up) (under $20). The next thing I would recommend is a biturbo exhaust (about $100). I thats not enough then go for the big enchilada - - a 70 cc euro speed kit ($300).