My brother bought himself a Honda Express NC50 2-speed from 1983. It was in a driveable condition, but was no fun. It could barely do 30 km/h, and forget about pulling a grown man uphill.
My first thought was to upgrade the intake and carburettor, but that gave only minimal results. I checked everything, and concluded that the 13 mm exhaust header pipe was the biggest restriction. So we ordered a MLM Street exhaust from the USA. While we were at it, we bought a Malossi 45,5 mm kit that was on sale from a danish dealer (Mopedspeed). It took a little grinding to fit, since it’s made for the Honda Camino PA50 model. But the transfers, exhaust angle, stud spacing etc. are all the same.
Now it rides really well, i think my brother will be happy.