Its a 2-terminal relay for a motorcycle, after adding an earth lead so I
could fit a car type; I then discovered that the intermittent operation was
due to a corroded connector block hidden under the tank.

After breaking up the epoxy potting I found the old relay was blameless.

The circuit could probably be adapted for 6V with a logic level MOSFET - but
would need 2 in parallel to handle twice the current (don't forget filament
cold resistance surge!).