First thing to check, as Beaky mentioned, would be the switch in the power shed - make sure it isn't stuck down. If it is stuck down, this would make the kicker go off as well as probably make the machine think there is an extra ball sitting on the switch if all 6 other balls are sitting in the trough. You would have to access the switch from under the playfield, it may even be slightly bent from years of use and might just need a bit of a tweak to get it sitting right again.
I assume that the CPU is good and there is no battery damage on the board anywhere?
