The current for a typical message was measured at 700mA, and I estimate that the maximum current will be around 2A with every LED lit. This gives the sign a power switch, an IEC cord, and the ability to take power from a wall socket instead of a bench supply.
The original supply has a standard form factor, so any supply in industrial form factor will fit in the original mounting straps. So the first step in converting a sign is to replace the 12V input power supply with a 120V input one. Vehicle electrics are a nominally 12 volts(*), and the original sign has a 12V to 5V power supply rated to 20 amps (in hand, in the image above). (NB: These instructions were created after the fact, so no ongoing construction details or images.)
