disable your oil injector first by running it into itself. you cant just remove it altogether, the air leak will be so massive your bike wont run. you have to either leave the unit on your bike or create a really good seal to cover the hole it's in with.
start running 50:1 premix.
take your cylinder off and clean out the ports. what ive found is any express ive owned had insane carbon buildup in the exhaust port which REALLY kills performance. if your bike is going 30-32 already, its probably clean, if its only hitting 25-28, the exhaust port is probably all clogged up.
clean the mating surfaces (watch out, OEM gaskets take a long time to remove, sticky stuff.)and put on new gaskets, and just make sure the engine is sealed up nice and tight and the bike runs about 30-32mph on the flat. start from there.
honda hobbit exhausts will bolt right up to your cylinder, however they wont mount correctly so there will be some hack n weld involved, but id recommend hobbit pipes mainly so you have a big header to start with too, instead of the TINY stock header.
hobbit cylinder kits will work well too, but youll have to grind some of the fins down to make it clear the oil drain plug.
somebody on here, or your local machine shop, can mill your head down to raise your compression. spree heads work too but they have minimal finnage and probably wont disperse heat as well.
hobbit reeds work, i think, in the NC50 engine. get some better reeds.
if you wanna get serious performance, swap out the shitty original one-speed NC50 engine for an 82-83 2-speed, or better yet, a variated NU50 (urban express) engine. variated bikes are the future! plus both of those engines have CDI units, while your original nc50 engine has points and the timing IS NOT ADJUSTABLE!
and of course run a colder plug after any mods you do!
there are NO bolt-on intakes for the express, but you could grind it out to make it bigger, and search around for a keihin/mikuni that would bolt up. somebody on here makes custom 19mm phbg intakes out of stock intakes. p.s....block off the oil injection plug on the intake!