How do I get my gauges to work?

Hose off your battery, and clean connections from negative to frame, to engine. That way we will have solid ground. Gauges need solid ground, sounds to me like the ground wire is not established correctly on this bus, so things tend to go wonky with bad ground.

You might want to check those fuses too.

I checked the fuses. Half of them are missing. How can I tell which specific fuses to buy to replace them?

the value should be printed on the circuit board....and on the cover. Use your good senses too, for example a radio would not use a 30 amp fuse, a horn would not use a 5 amp fuse...like not hard.

as I recall the Datsun had blade type fuses with the value printed on the side of each...not as handy as the glass type, but the thinking is with a probe you can test them (the spine has the connections exposed) without takin' 'em out.

replace all missing with like 5 amp .if not heavy enough will blow out,then go higher on that one ,like 10 amp if it blows a short is somewhere

Speed is a mechanical connection. Take the dash out to see if your cable is broken and to check if all the other dials are plugged in. There is a block of connectors under the passenger side dashboard. Those all connect to the dials in your instrument cluster. Make sure all of those are connected too. And do replace the missing fuses. Datsun Factory Service manuals are easy to find with a google search. I think xenons30 has them.

voltage regulator, ignition relay....check power to harness that feeds the dash gauges. Check your turn signals..i believe they run off same system to if they work, might be another issue.