Steveg,
I think that there are a variety of upgrades that they do. The one that I have seen uses one of the tuning coils (the oscillator coil) from the old chassis to tune the stations. This must be a fairly specialized piece of electronics to do that. The dial would work fine for the AM side but I don't know how the FM is handled and I am thinking about markings here. Some of the conversions use a type of blue luminescent dial that only shows up when it is turned on.
It wouldn't be hard to do but sourcing the modules would. Part of the reason they are expensive is all the labor involved, cleaning and refurbishing the case and buying all the new front parts.
