In addition to the FS-4 previously mentioned here, another small company used to
make an analog VFO kit that accomplished nearly the same thing. It plugs into a
crystal socket, and it contains a separate 500khz crystal calibrator. You use the
calibrator to set the VFO to the 500 khz segment you wish to receive, and then it
works just like a crystal. It runs from 12VDC. I think the guy who made them is
K9ZMU. They are of course not as stable as a crystal or synthesizer, but it is not too bad, and certainly good enough for listening to short wave broadcast stations.
Note that there tuning ranges that do not work well even with the intended Drake
crystals because of IF mixing problems, and that problem is much worse if you intend
to use a substutute device to control an associated T4X transmitter.