cams
I wouldn't bother you'll do more damage swapping it out than it's worth.
Better of getting a decent exhaust made up, or just tinkering with the air filter and carbs. Anything you do with stock engine isn't going to make any difference.
Just make sure you have some good plugs, decent leads, new dizzy, rotor, good engine oil, new filter, new air filter, etc etc.
Biggest mods to make a difference is the exhaust add 10bhp and 15lbft with a complete downpipe and system, having a header probably gain you extra 5bhp and 5lbft, will increase mpg as well, then only other thing is headwork, tweak carbs, fresh gearbox oil, lightweight flywheel, make sure you keep stock wheels as nice and light, fancy looking new ones will weigh twice as much reduce mpg and slow you down accelerating.
Removing excess weight is next best modification but that's down to how extreme you want to go, stock seats pretty light, but you could race pin the bonnet and boot lid ditching the hinges and locks etc. remove sound proofing, removing all the extra nuts and bolts on everything as Honda love to go overkill on those. But just removing spare wheel and tools is quite a bit just carry can of tyre glue and electric pump (lighter than foot pump! lol!)