Uhhh, I dono John, but if you feel an XR600 is needing more power,,,,,,,, uhhh, maybe its time to to go see the Ducati Dealer for a 916 ? :) Good luck!
Already have the ported big fin head, white bros. 1650 camshaft, upped compresssion, large diameter headers,
Pro Circuit exhaust, K&N. A flatslide is also on the way.
Not to sure about a 600, but here they have a XR650 with a HRC power up kit, i havent ridden it or anything but it sounds good with the yosi pipe, and with a K&N filter, but apperntly you notice the difference, its ALOt, they had a write up of it in a local mag here saying it was great and all the rest.