Not starting after being warm 1973 Westy

If it does it again.
Turn the key on.
Then turn it again as if you are trying to activate the starter.

When you have the key in starter motor mode
Look at the alternator and oil light on the dash.
If they go dim this is telling you that power from the key is getting to the starter solenoid.

If it wont turn over you need another starter.

If when you have the key in starter motor mode and the dash lights don't dim either look and see if the black wire has fallen off the starter solenoid or
you need to replace the plastic electric block that's under the ignition barrel.

 

^this. And fit a hot start relay as a first step, as Dicky says. The starter solenoid can draw 30amps (or more) pulling in. If there's any resistance in ancient/corroded wiring, you'll lose valuable volts. The solenoid won't pull in, so the motor won't start turning.

It's unlikely you need a new starter.

 

Mine needed both a new ignition switch and then a starter.
But then it used to have cold start nothing hot start always problem. So once it was going it worked for the day every time.

 

ALso relays are cheap!

 

But if they're Chinese, keep a spare!