Problem with Kawasaki Ninja zx 7.
I have a '91 Kawasaki Ninja ZX 7. I've owned it problem free for almost two months until last week when i noticed a problem started to happen (as far as i noticed it started all at once). The bike rides like I'm holding the clutch half way in (the engine will rev but it wont go much faster), and it becomes worse the faster it's going. At lower speeds, 0-30mph, it's not that noticeble but after that it gets proggesivley worse and at 70mph it's so bad it wont go any faster (when I hit the throttle the engine revs like the clutch is in and wont go any faster). After this had happened i changed the oil, cleaned the air filter, and put in new spark plugs but the problem still persists. The clutch looks ok (although I didnt excactly know what to look for) but I figured it wasn't that because of how it gets worse at higher speeds, and if it was the clutch it would probably be the same, no matter what speed. Thanks for you time, any help and I would be most grateful,
p.s. I don't know of any previous repair history on the bike. I know the previous owner bought it used and raced it a vintage race. He did tweak the engine a little to make it faster, although what he did excactly i'm not sure.