Send your cam to megacycle for a regrind, is probably number one.
A big bore kit -- 550 or -- if you're feeling lucky -- 600, can be had on Ebay for pretty cheap, although you'll spend more money reboring and such.
The CR carbs are reputed to be worth about 5 HP themselves, but that's more than $100 per pony.
Porting and flowing; some poeple say it's magic for the SOHC, others say it's a waste of money. Couldn't say myself.
Good quality pipes supposedly unleash these bikes, but I don't know how anyone ever found that out as I've never seen anything but simple "old tech" Kerkers and such.