The 07 crf50's dont have a air mixture screw on the carbi, as the older 05 bike do have the mixture screw. other than that not much differance.
Ive got a 07 crf50 myself, i just used some very smal washers to space up the needle clips position to tune the carbie in better for my use, but if your getting the 88kit straight up it dont matter what year you get because you'l be needing a new carbi anyways.