..if you ordered it from kymco, then it's probably the right one...speculating, but it probably is...
..i would definately go with an aftermarket..such as malossi, polini, NCY...
..and though i'm not sure of the OEM of the S9ac, a 1000 spring would breath new life into you S9...
..if you do many mods, a 1500 might even work better, but since yours is air cooled, i'd put my money on the 1000...