I got an 05-06 rr head for my 03 636, it has K2 stamped on it. My 03 636 head had adjustable cam sprockets. I think you're better off comparing valves so you know for sure. You could go for an 05-06 rr head with cams if you can find one, it has race cams and ports reshaped, stiffer valve springs for higher rev limit and bigger valves again. Or tune the ignition and injection, degree cams, porting, balance, blueprint etc. What you do depends on budget. Most do full exhaust system, race filter, PC 3 custom map to start with. Maybe lighten the bike to get more out of what's there, often cheaper than engine work.