I don't see much space on, under, or especially behind the tiny dashboard on these (YM186), for gauges. I know disconnecting everything under there to pull the dash and replace the steering box on the YM186D I sold, was a nightmare because everything is jammed in so tight.
Anything is possible but my own preference is avoid complexity when I don't see how it will add value for me. These models are reliable and any warning is unlikely.
I'm pretty sure I would notice a sudden new light sooner than noticing a gauge reading out of spec.
We see a lot of new owners appear here and post "my tractor is running hot and the warning light comes on often. I need to install a temperature gauge!" Uh no, bring the cooling back to spec - get the radiator rodded or replaced and it will never overheat. Same reasoning in this case where the discharge light no longer comes on at idle. There's an electrical problem and adding more complexity to more precisely measure amps or volts isn't the solution.
OP, the discharge light should be on at idle, then weaken and die out as you get up to about 1,000 rpm. When you get the wiring (likely a ground) connected as intended this light will provide all the information needed. The other lights will come on momentarily as a test each time you crank the starter.
it will be years before you need to do anything else.
