I did the same conversion last year, I bought a 03' cdi, 900 powervalves, but cutting them is easier and cheaper, you can also shim them out with washers. I also bought the 900 y-pipe and stock exhaust cheap, like $75.00 for both. I was told that I NEEDED to have the y-pipe.
I talked with Kelsey at RKtek cause I have his head and mentioned that the zr800 pipe would work, try getting ahold oh him at www.Torquelineproducts.com
. Changed the gearing to 41/26 ason the 03' 900 and added a 1.5" track. Couldn't find anything I couldn't beat when finished.
Sell your 800 topend for $400.00 os so along with your cdi and save some of the money you spent on the 900.
I had a Black Magic Single on it, but it ran bettter with the stock 900 pipe and PPP can I have from my 800.
I also changed the carbs to 03' 900 spec's but put a different needle to handle a lean midrange on the 03' 900 mapping. I could that needle number if you need it.
Like I said, I could out run any F7 and REV I could find.