The voltage regulator should put out 5 volts, if I'm not mistaken.
I replaced mine with a solid state.
The gauges (oil, fuel and temp) have a bi-metal strip that is wrapped in a wire that has a little resistance built into it. When a current flows through, it will heat up and bend the strip. This is what moves the needle.
I made an attempt to replace my wire on the oil and temp. And it actually worked. But not perfectly. You have to find a wire with the exact resistance, otherwise your gauges will show incorrect readings. (My problem)
Reproduction gauges are out there for ~ $80
I would go that route if I were you.