Using anything OTHER than a real arcade monitor in a JAMMA cabinet would cost more.
20" is not a standard arcade monitor size, unless you are in Australia (only US game I can think of that had that size was Turbo (upright, and cockpit only).
19" is THE standard arcade monitor size, and they are pretty cheap used. Look for one of those, and don't pay more than $50 - $100 (remember, you can often buy a complete working Jamma cabinet with monitor for $100).