The typical 1964 replacement for A/S is the American Standard Glenwall.
It mounts on the same hardware fairly quickly.
The Crane Rexmont is a piece of junk.
Inwall carriers require a lot of work and plumbing. New drywall too.
Changing to floor mount requires a lot of work too.
I'm not a big fan of the pressure assist toilets, the Glenwall will shred the paper and other things a bit, but is very plug resistant.
Without dropping big dollars, it's the best choice.
American Standard Glenwall with pressure assist