I have been looking for replacements for the front speakers in my '70 also.
I have not found any. The new speakers have magnets that are too big to fit in the left side position without interfering with the instrument cluster. However, I have heard that some speakers can be rebuilt, but that is expensive, and their quality can be only as good as they were new, which is not good by today's standards.
So long as new ones fit, and the impedance is the same, I can't see why new ones can't be used.
Mike