Thanks for the information, looks like this has been discussed here before.
So the main considerations in finding a spare would be:
1. Bolt pattern
2. Tire height and width to work with the sure grip and fit in the front wheel well.
3. Offset to clear brake components.
4. In my case a normal P or even an LT type tire in lieu of a "spare doughnut" type would be more durable. Just having one in the garage a mile or so away would eventually get us home (after a short hike back).
The original space saver is in place but being 44 years old it may not be a good idea to subject it to a gravel road, probably OK for paved roads though.