even with a gearbox youre pushing it...
strangely, a smaller roller might help... too big and it slips and wont ever get a decent speed. just revs and shreds tyres.
being a fourstroker, there isnt much you can do to improve on the honda design except by spending money...and reducing life. on that note, theyre designed for longevity and usability, not flat out power.
pretty sure theyre OHC, so you could get the cams reprofiled. make some more grunt up the top, lose some grunt on the bottom end. good luck finding someone that can do it on something that small. then have fun sourcing stronger springs for the valves, increased wear on running surfaces, etc... but thats the one major mod that will give the best results. maybe a model machinist/mens shed type place can help. guy round here grinds his own cams with not much more than a bench grinder with a fancy jig bolted on the front. google
never had a look inside my 25, but i daresay even thailand built hondas have pretty good ports. as long as you know what youre doing, theres no harm in experimenting with opening the ports up a bit, improving flow(or even reducing them for "velocity porting" which is actually a better way to go. yep. get more air into the cylinder by reducing port sizes. strange, huh? crash course in fluid dynamics...).
intake length can make a difference on the torque curve, as can getting the exhaust as free breathing as possible(and tuned...yes, you can tune a fourstroke exhaust pipe). which would require a new carby, which you can possibly go up a size or two with(to match that bored out intake port...). but you will need one with adjustable hi-lo jets. fiddle round with new throttle cable mounts, etc... the barrel/ rotating carbs are great for convenience but i havent found any that are (fully) adjustable yet. standard butterfly types usually have no dedicated cable mounts.
find die grinder, tear it down, modify it to be driven from a belt, use it as a supercharger...
ignition is fixed but you can wriggle the coil around a bit, file its holes out into slots, play with the few degrees change you can get that way.
be happy with 30MPH on a pushbike? it gets you from a to b. and back again.
tyre diameter wont affect anything, its a friction drive.
swap it out for a 2 stroker... my lil husky from years back used to hit 60km/h...about 40mph? with a roller diameter of about 13/16". tried 1", just shredded tyres.