No easy conversion because the scan rates are so much different.

Best to find a used RGB input video monitor to use or fix the old one.

The video capture cards do not even have RGB input but typically only
have composite or s-video inputs, some newer high end ones have
component input, but no RGB input.

Some of the earlier multi-sync VGA monitors that had the bnc
connections also would do the standard ntsc and pal frequencies, but
those are rare to find now.