The video signal from WoW and GORF is basically Component video (YPbPr), so you could probably use the YPbPr input of a converter like the GBS-8200 (haven't personally tried though, so maybe the levels are different).
That's not ideal though, since it converts to VGA, so you'll need to install a VGA monitor, and you'll have the slight lag added from the converter. If you're gonna do that, you might as well find a 19" CRT TV with YPbPr input and swap it into the cabinet.
I'm not aware of a converter that takes YPbPr and outputs 15kHz RGB, like the WoW/GORF converter boards do. It is a pretty simple circuit, so in a pinch, you could build your own. Simply cloning the original possible... the only IC used on the board (TBA530) is obsolete, but still available and pretty cheap. A new design probably wouldn't be too tough either.
Or, just put out a WTB, and maybe someone will sell you an original board.
DogP