When I renewed the C of A for my old Beagle Terrier back in 1995-ish, it was inspected by a CAA surveyor. He picked up that it had never had the 'starter warning light' fitted!
At that time Aircraft Spruce sold a little warning light and sensor which you simply added to the busbar. It sensed busbar voltage and so with a starting load it would come on. Equally it doubled as a 'low volts' warning.
Although uncertified our surveyor was happy with it. shame he did not look at the quality of the other work - the exhaust literally fell off and head gasket failed within 5 hours!