have you read that? It spells most of it out pretty well
there's all the little things that make little differences, but can be significant: possibilities your timing is off, decomp leaks, definitely sounds like jetting could use improvement, making sure the pipe isn't clogged, and checking that the brakes don't rub everything spins smoothly, chain not too tight etc.
Next up, pipe it. I think you could regear for more torque and let the pipe enable some higher RPM to maintain similar top speeds. might hurt reliability and fuel economy but probably worth it.
Then D. D topend is a decent lil boost, D crank along with it makes a lot more torque, with a pipe it's a fantastic setup for reasonable speed, torque and reliability.
An Airsal kit just slapped on there would be pretty easy & satisfying and a big improvement too all around. even without doing much else but tuning it and maybe opening the stock exhaust so it doesn't overheat, though it's a little bit more of an invest cost-wise, kitting it is probably honestly easier than swapping the sprockets to regear even... maybe not quite