Most of the gauges are mechanical. Oil, Speedo, Temp are all mechanical. Fuel gauge is not voltage sensitive, it is however current sensitive so most folks use a "Runtz" type voltage reducer to protect the gauge. The Amp meter won't care if you run 12 volts or 6 volts. It measures current. To convert from 6 to 12 the minimum needed, is a Battery, Generator, Regulator, Ballast Resistor(for the points), and light bulbs. A 6 volt starter works fine on 12 volts, just don't let it get hot. Light bulbs are interchangeable as far as the sockets are concerned. Dash lights fail, could be a bad connection or a broken wire or an issue with the headlight switch. The dimmer resistor might not like the load with the dash lights.
