Need advice on what to fix

Do you mean nothing else works? No head lights? No interior lights or radio? What history led up to this? There's a number of fusible links for various circuits, but it's unlikely all of them are blown. Did this problem occur all of a sudden?

Dandy. I guess we can pick any circuit to start with. Best is to use a test light. For these tests that will be more accurate than a digital voltmeter. Start by testing for voltage between the large post on the starter motor and a paint-free surface on the engine block. The test light should light up brightly.

Leave the probe on the starter and move the ground clip to a paint-free bolt head or some other point on the body. Test light should light up again. Turn the head light switch on during these tests, even if they don't work, because that will cause current to try to flow, and that will make some defects show up easier.

From this point you can leave the ground clip on the body. Check for voltage on the large, bolted-on output wire on the back of the generator. Let me know if any of these voltages are missing. If they're not, we'll have to look at the next group of circuits.

No. That would be too easy. There's a number of fuse link wires.