The later (1975) solid state assimilator (P/N 066393) has connections for both legs of the alternator and a switched (white) voltage wire, a lead to the light and a ground, where the original mechanical has only one leg of the alternator, a lead to the light and a ground, so you will need to run a couple extra wires.
The alternatives are a simple voltage switch which wires between a switched negative lead, a ground and the light. These generally turn on the light when the voltage in the wire drops below 12.5 volts. These are usually small cubes that can easily be mounted inside the headlight shell and are less fragile than the mechanical assimilator and work at least as well. These are available from several shops such as RGM and Al Osborne, but not as easily found in the US.
Of course an ammeter or battery status (voltmeter) monitor will also work if you don't mind finding a place to mount them.