Sorry to hear someone took the power supply you were using. In the shop I used to work in we'd sometimes have people "borrow" tools or equipment and not bring them back. We setup some cheap security cameras and that stopped the nonsense.
Not sure why the video converter doesn't work, but since there are different video formats maybe the CalOmega video signal is not compatible with the video converter. Sometimes the sync pulses are opposite from what the converter is expecting (i.e., the CalOmega sync pulses might be negative and the converter is expecting positive pulses, or vice versa). Or it might just be a pinout difference on the CalOmega video connector. Back when the CalOmega mpu board was designed and built they didn't have the same video standards we have today.
If you can get info on the monitor or monitor board that the CalOmega machine uses that might tell you what type of video signal it expects, and that would be the video signal the CalOmega mpu board outputs. If you have the schematic drawing for the CalOmega mpu it may show what pins the RGB video signals are on, as well as the sync pulses. Some mpu boards have a dipswitch setting that selects positive or negative sync, for use with different monitors. You might look into what the dipswitches on your mpu board do.