Stock chappy does 23mph on the flats if you have a fast one! MLM pipe alone got me to 28mph on the flats. You can also modify the stock pipe to make it less restrictive. A chappy I bought, someone cut the muffler, I assume removed the baffles inside and welded it shut again. The spark arrestor is removed too. I had quicker pick up with that mod but it topped out at 27mph. Also note it was about 30% louder than stock. The MLM pipe is only a couple of decibels louder than stock.
I also just put on a lightly used Polini kit with MLM pipe on my son’s chappy with stock carb and it actually got slower! The issue is the carb is way too small and not tuned for the bigger cylinder and free flowing exhaust. My recommendation is to put effort into tuning a bigger carb instead of wasting time rejetting the stock carb. Options for bigger carbs would be LB80 carb or pw80 carb if you want to keep the stock intake. Not sure whether the pw80 carb will be plug and play like the LB80 carb as the main jet is a 120 vs 90 main jet in the LB80 carb. Mikuni VM18 or VM20 or Dellorto 19mm, are other options but all require aftermarket intakes - MLM or Polini intakes.
Last to consider is if you want to keep the oil injection. The LB80 oil injection has a different size plunger within it’s pump than the LB50, so it puts through a little more oil. Not sure how much it matters with today’s oil technology as so much has changed between now and then. You probably get more protection with a little less oil for today’s oil vs that of late 70’s/early 80’s. You can also remove or disable the oil pump and premix.
Good luck and anybody else feel free to chime in.