Typically, can ham-bands-only receivers such as the Drake R4 and Collins
75A or 75S series, transmitters such as the Collins 32S and Drake T4
series, and transceivers such as Drake TR-4 and Collins KWM-2, be modified
to work on CAP and MARS frequencies?

I know that some radios have provisions for installing crystals so that
they'll work on off-band frequencies.

How can I determine which radios are modifiable that way, and also where
is a good place to get crystals these days?