[flatheadmurre]

Voltage reducer wouldn´t do you any good in this case.
A 12v voltmeter usually starts at a voltage to low to really be any good in a 6v system.
If the voltmeter starts at 7v it will barely show any movement until rpm is up and generator charging.
Anything can be rebuilt and converted...but it´s probably less of a pain and cost to keep searching.