Horizontal Scan Rate = (Total Vertical Lines per Frame) x (Frames per second)
31kHz = (480 visible lines + 45 blanking lines) x 60Hz
so the following is also true:
31kHz = (576 + 44) x 50Hz
A 15Khz CRT will go from about 50-65Hz and same for a 31kHz arcade CRT, i believe. (A PC monitor is often a bit more flexible, and will probably sync anywhere from 31kHz up to its horizontal scan limit (google for the specs) at anywhere from 40-160Hz, as long as the equation balances. You can also look into something like 120Hz 240p (or 100Hz 256p))
EDIT: oops, section removed as that's not a PC monitor you have
The only problem with the above might be that some old arcade monitors can be sensitive. Keep it on spec if you can.