It's also extremely common to simply have a bad connection due to surface corrosion between the light bulb sockets and the circuit board. Remove the bulb retainers from the back of the circuit board, shine up the connection points with a bit of emery cloth, confirm the bulbs are good and glow on the workbench, and reinstall.

Alternately a missing or broken circuit panel ground wire will fully interrupt the circuit and any possibility of lighting. This is another very common reason for failure.

A test light or multimeter are your friends for this sort of thing. Confirm 12-ish V at every step of the way (while lights are on...with dimmer set full bright), and wherever the 12V disappears is exactly where your problem is. Much faster and less frustrating to troubleshoot like this, than it is to go the trial-and-error of each part method.

You will need different style of sending units on the engine for temp and oil pressure, if you switch from your current dummy lights to gauges. Dummy lights operate on sending unit on/off switches, and gauges operate on sending unit rheostats. Not difficult, just additional parts to replace so the gauges work. And the replacement gauges are integrated with the circuit board I'm quite certain...so if you're fixing lights on a circuit board you're not even going to be using, due to a future gauge swap, you might be spinning your wheels on this lighting project a little bit. So I say swap your gauges first if that is your intention. THEN worry about getting your lights working.