The simplest to use test equipment for your sort of problem is a 12 volt test light.
Available on line or at any auto parts store. $10-15.
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Connect the alligator clip to the battery ground. Use the sharp end to check for voltage when its internal light will glow.
Start at the fusible link and then to the key switch.
However, if you are able to crank the engine using the key switch you definitely have some circuits working.
Are you troubleshooting from a wiring diagram. Here is one from the Operator's manual.
I would be looking closely at the various connectors in the wiring harness for corrosion, signs of over heating or wires burnt off.
A cheap multi meter to check charging voltage when the engine is running would also tell much more than a light not coming on.
Dave