For the electrical system I'd suggest a modern voltage regulator and tossing the old rectifier and regulator first. Then, if it hasn't already been done, connect the Yellow and White wires in the headlight bucket to get full charging all the time. There's also an upgrade stator being produced by Matt, aka Sonreir, from Sparckmoto that will bring the charging system wattage up to the 170W area so a real headlight can be used.
Engine wise? before any mods are even thought about the engine needs to be up to par with compression, leaks, etc before hand.
If you look at the Horse Power output per CC you will discover these are pretty much high performance motors right from the factory.
Most of the Racing mods are for running at extended max rpm ranges for extended time periods.
Lighter Valves to reduce bounce
Stronger Valve Springs to reduce bounce
Slight Cam Mods to improve the TOP End with considerable loss of Low End rpm performance.
A proper well built and setup STOCK MOTOR will provide you with a LOT of fun and be much more enjoyable then modifications.
Unless you like to spend most of your time making consistent tweaks to always dial in the things you sent south by other modifications.
Remember every change has implications in more than one place.
The Honda engineers spent a lot of time with these and they were the longest running production motor of a single design.
That should say something right there.
A forum community dedicated to Honda Twin cylinder motorcycle owners and enthusiasts. Come join the discussion about performance, engine builds, classifieds, modifications, troubleshooting, maintenance, and more!