how disable/ turn off cobra gold line alarm?

Alarm was installed post purchase of vehicle in 1992. The alarm had been disabled using the under bonnet key to the system box. Recently replaced flat battery - now alarm permanently sounding off 4 - 5 secs then off and repeat. I think it must have reset with the new battery install to ON. Have misplaced key to immobilser unit under the bonnet and the 2 radio keys gave up the ghost years ago there is a small led light lit in the centre console. How can I disable or get get replacement master key/ radio keys