Leaving aside the fact that such an unnatural conversion to LCD will almost definitely give you chronic halitosis and make your teeth fall out, no really...
Does the chassis have a horizontal hold pot and have you tried adjusting it? Have you tried an alternative video source that the CRT monitor can accept?
What resolution and signal format is the game board supposed to output, and it is actually doing so? Does that resolution line up with something your converter board can handle? I would imagine most don't like 24kHz, for example.
is It's possible the game board isn't outputting video correctly