Sounds like there is a compressor driven by a v-belt. This was a pretty standard setup years ago. Could have a TX Valve as well. If you want it to run off 12 volts and use the existing evaporator plates (or eutectic tank) you might be best talking to a refrigeration mechanic to recommend a suitable 12V compressor. Another way would be to rip out all the existing refrigeration system and buy one of those kits you see advertised and fit it yourself. $$$ no matter which way you go.

The old system must also have a condensor somewhere. This can either be a fan cooled or water cooled.