Nathan it's about the same as for an OHV engine. Headers make the biggest difference, then increased carburation (390 CFM is about the max anything but a full-out race engine can handle), aftermarket heads, cam, ignition upgrades, etc. Then it's a matter of fine tuning what you have.
[updated:LAST EDITED ON 22-Nov-02 AT 08:56 PM (EST)]I would say the best, besides the cam and other such items, would be to upgrade the oiling, porting, and ignition. The oiling would do the most good for due to the fact that it really doesn't filter anything at all stock. One of the Crane Cams XR 700 electronic ignition's shutter wheels fits on the newer flatheads distibuter, haven't seen if the eye would fit but it should. Don't know a thing about the Petronixs, but they might fit too. The electronic ignition would improve responce (along with the other appropriate items), and I believe responce is the most important thing for an engine to perform well. There are also some well developed methods of porting the decks, heads, and ports of these. But be careful about doing that, I would only do that myself if I had full knowledge of what I was doing, and if I took it to a shop to have it done, make sure they knew what they were doing also.