I don't think it will work. "100 Series" generally refers to the PowerBooks 140, 145, 145b, 150, 160, 165, 165c, 170, 180, and 180c.
To be very honest, I wouldn't invest a cent in such an old machine, unless it is of particular emotional value.
If you are still interested in buying the modem, you might have some luck with posting in the Buy/Sell forum. Maybe someone has one lying around...
tooki