If it were me and the gauges were working perfectly (other than being darker at night), I'd rig up 2 tiny LED's as "license plate lights" like a previous poster mentioned.
You may even be able to integrate a small LED (not a bulb, just a diode, the size of a grain of rice) into the gauge itself or laying on top of it, etc.
2nd option was already mentioned.
Gottaluv the headlamp!
Replacing with digital gauges (even it were possible) seems like a mighty bit of overkill and potentially complicated. ie: is your tach mechanical? Can you match the resistance of your fuel sender with something digital that uses the same range of resistances? If you can't answer these questions (and no offense intended), then the previous 2 solutions are the ones for you. I doubt very much there is a "plug n' play digital solution for an old tractor like that)