Just wanted to add two cents... Kawasaki has PNF, positive neutral finder, it won't go into any gear except 1st and neutral if it isn't moving and the trans turning a certain speed. I realize your shifter may have other problems, but this might be handy to know once you do get it to shift. Not knowing this helped me get a better price when I bought mine. We both thought there might be something wrong with the shifter and dropped the price, turned out to be nothing wrong with it.
I spotted one of the 2-stroke bicycle kits in an auction the other day, starting at $5. (up to $38.99 now) Thinkin' I should convert my Mongoose for when gas hits $5.