If its the factory seat, you may want to look into finding a corbin or buell pro seat. These will still both be narrow. You could also send you seat of and have it reworked.
Another option would be to install an M2 tail on the bike with a M2 seat which is much wider. Im thinking a 98 M2 tail will fit an S1 without much difficulty, not sure about the 99+ M2. After saying all this, im not sure about how much wider a 98 M2 seat is, I know they changed the frame to the later frame on the 99+ M2 and the 99+ has a wide seat , im just guessing the 98 and 99 M2 seats are the same, not 100% sure though.