my recently purchased 73 1.7 with rebuilt engine won't start today with temps in the 40s. What is the problem?

That's not much info for an intelligent diagnosis.

Does it turn over? Do you hear the fuelpump running ? Is the battery ok ? Do the intrumentlights show when you turn the key? Is there gas ? Etc etc.
It is problably not related to the temp so please provide more info !

Fuel injected or carbs?

This sounds more like a quiz question than a real question. If I have a real question I check my thread regularly. Yes, I know you can subscribe to your thread, but emails from the forum do not always come thru immediately.

? Got battery ?