I'm not sure where a person would go to buy onesies and twosies of the
required tuning capacitor and transformers, except by scrounging at
hamfests and the like.

I've been very happy with my McIntosh FM tuner, and had figured on
eventually picking up a mono AM/FM Mac. Nothing against having one which
includes the multiplex adapter, but I figure folks will be more willing
to make me a deal if it's missing ;-) As the various disc and mica caps,
carbon comp resistors, original wires and connectors seemed to be in
good shape, I left them alone. Electrolytics and paper caps were
suspect, and the selenium rectifier was weak, so I replaced these with
modern audiophile-type parts, and all's been fine. Some folks carefully
pull the guts out of vintage caps and restuff them with modern parts in
order to retain the vintage look inside and out. But there was no way I
could do that when replacing 10 uf electrolytic coupling caps with 10 uf
Solen film caps!

But if I were really *really* serious about wanting the best possible AM
for all situations, including DXing, maybe I'd look into a "boat anchor"
general coverage receiver from the past, and combine that with a modern
synchronous detector with selectable sidebands. And I'd ensure that it
offered at least one wide-bandwidth setting.