I've read some interesting threads about QT 50 mods but the single most effective thing I believe you can do for a QT 50 with any kind of mileage is to overhaul the top end.
After tinkering with these great scoots for the last couple of years I've learned through trial and error that once you put a new piston and ring set in (and wrist pin bearing if you need it) that most of the idle demons and poor performance stuff goes away. Great surprises in mid throttle and top speeds of 30 to 35 are not uncommon on the flats (even carrying me at 185 lbs and the wind in my favor).
If you are going to do any mods, this is where I would start fooling with the air box drillouts and exhaust port work.
The one thing that I like the most is how quiet these things run once you've replaced the piston and rings. I've got 2 runners right now and both have stock mufflers. Since the rebuilds (#1, new stock piston and ring set, with only a good hone job, #2, new 1 over piston and ring set with a bore and hone job) I've lost the piston slap or clatter, totally. The only thing I hear is the wind in my ears and the whine of the transmission and pinion gear. THESE QT's are tight.
Anybody working on these scoots should really consider this. It's fairly easy. It gets a little pricey if you have to bore the jug but well worth it.
ATLANTIC BEACH, FL