Yeah the thread is old like me (45) anyway I have a 1980 FA50 an old bike with low miles (2,200 mi.)and I have been trying to get more top end from this bike, on the stand the bike would not tach out and on the road it had an issue with not topping out. I doubled up on the air cleaner element and cleaned the fuel system, all of it twice. what tilted the cards in my was when I cleared the carbon out of the exhaust port and there is a little chopped hose on the side of the carb it's a float vent, when I plugged it the bike reved up higher go figure, as for the 70cc upgrade I came across a site I need to do some searching to find it again they are available from $97.00-$168.00.
I'll get back to ya