EARLY A booster

install any 73 up booster off any ABEFMJ car line, that has the same mounting pattern. Then you'll probably be wanting to make it easier to modulate which just a matter of swapping reservoirs.
Before you reinstall the MC, measure the bore diameter to the nearest 1/16th inch.
Caution: make sure the pushrod is NOT too long, or you could break the control valve on the first hard stop.

But before you do anything;
make sure the rear brakes are not dragging. And make sure your current MC is correctly plumbed; the frontmost reservoir should go to the rear brakes.