The CV's scan doubler is actually just a (clever) way to pipe the RGB output into the VESA feature connector of the S3 chip and then use a digital overlay to show it through the VGA output. It uses an A/D converter and some circuitry to adapt the signal - definitely quite a bit of work to rebuild, but probably possible. Since there's nothing comparable, there's little chance of replacing it with something else, sorry.
But: you can use any other scandoubler independently of the CV, but you'll need a 2nd monitor or an external switch.
