do you have the assimulator to reuse ?
which type ... there is more than one style .
and how important is " original" for such a marginally effective piece of equipment .
the original red warning light comes on with the ignition
and the assimulator ,
... opens the dc connection , with a certain amount of ac output .
( a simple relay function )...
using one with 3 phase would somewhat skew the bulbs on/off readings .( I'm pretty sure )
... but accuracy and efficiency are secondary to the simpler function of idiot light .
the bulb may flicker at idle before going out ... as the RPMs increase .
and fully light when headlight is on at idle .
( this would be either super active and handy ...or super irritating ? )
there is an earlier , simpler mechanical style ...using toaster wire to over heat and break the connection .
similar to an overload thermostat or blinker-thing with no blink . ... ( lives in a can on a spring )
... and an improved Style that uses a transistor .
if you dont have one in hand , I wouldn't bother .
( but the headlight shell hole can be reused more effectively )
these days there are options
both more efficient and practical ... and probably cheaper.
LED battery charge indicators ... with 3 color displays
the color changing with voltage
and adding blinking colors to indicate critical over and under voltages .
the wiring of these is also less complex .
2 wires , one wire to ignition switch power , the other wire to earthed .http://www.sparkbright.co.uk/sparkright-eclipse-battery-voltage-monitor.php#
is just one of them . ..