Something I never heard very much when this question is asked
Electrics.... If they are top notch that means better power delivery to the coils and points or electronic ignition. Lights and switches. Switches are notorious for power loss
Go through the whole loom, wire from plug to plug or bullet connector. Check condition by continuity checking, remove pins from connectors, check the joins for corrosion or damage. Clean, fix replace connector if it is breaking down. Tedious. but go through every wire. After cleaning go through to the next connection. Do this up to all or any switches, brakes or blinker relay, then continue past, again continuity checking each connection.
When the circuit is done then include the switch or relay by continuity, to see if they are working efficiently..
Remove, dismantle all bar and ignition switchs, clean check solder joins (common for these to have wires loose or strands of the wire detached. Again clean all contacts and lightly grease, ( I use a lithium based white grease), assemble and replace.
All earth connections need to b removed cleaned, some fine sandpaper, making sure the connection to the frame is good then a light contact grease, or I prefer to use nothing then smear a good coating on top.
This alone with a combination of the right coils and plugs will give better fuel economy and that equates to more power. Not much but it all adds up.
For some reason I couldn't copy and past the link. Screen shot of crusinimage 2mm, ( around 190cc) oversized pistons.
Cheap at $80.00+ postage.
Clean up the ports,casting marks, a valve grind, a good electronic ignition coupled with good coils that deliver a long duration and iridium plugs.
Some go for VM carbs, they can b expensive + a bit hungry on fuel. EX500 carbs give good response and those that have them set up properly say they are better than the 650 CV's although they are still a CV carb so will work if changing altitude..
The hardest part is getting the ignition. Bout the only one these days is Boyer and this will eat into the budget.
All this will give a noticeable gain within or around your budget