Surely there is a shop with knowledgable techs in England. Over here in the U.S. it's Bob Ginder in Tennessee. http://bjracing.com/
I rode a stone stock '74 to a AHRMA National Championship (Novice) in 2003 and during that time I had the opportunity to ride Bob's personal bike. This machine was like cheating. The power and deliverywere silky smooth and cleaner response across the band. It has head and port work,electronic ignition, exhasut etc. Bob knows his stuff and "someday" I'll send the head and barrel for the mods.