2009 R6S losing power and speed over time???

I recently bought a 2009 yamaha r6s with roughly 27k miles and have been riding it for like 3 months now, as ive been riding i noticed my power slowly dropping off, in the begining i was able to reach speeds of 160mph rather quickly but now the power completely drops off at 130-140mph even though i have like 6k more RPMs on the techometer, my first thought was a valve adjustment or maybe a tune but i cant find a tuner for an r6s (if you have any recomendations lmk) but the valve adjustment was quoted at $800 and i think that is absurd, my issue isnt that i cant go faster but that there is something limiting my bikes potential, it is straight piped but untuned, I also just recently replaced the chain and sprocket bc that last one was missing like half the teeth on the inside. any help would be greatly appreciated just want to make wure i dont get scammed or get a diagnosis on my bike.