spice, that's not refered to as a pump, it is a refrigeration compressor
You can't test it for functionality without having it hooked up to the rest of a system, so you can actually find out if it is compressing refrigerant and maintaining correct flow and pressure. All you can do is see it it runs at all, versus being frozen up. (Nothing to do with cold, but with being seized.)
And the first thing you need to do is find out which refrigerant (R12? R134a? R22?) the compressor
is rated for, and which one was in your old system. You can't just mix 'em up although you can debate about flushing
and converting them.
A system marked "spare" should be plugged or capped if it isn't fitted with self-sealing vlaves, because any moisture in the system mixes with the old freon and forms acid that eats things out fro the inside. That also applies the the parts on your boat.
So, yes, a spare compressor
assembly MIGHT be all you need. If it matches up to the rest of your system, if both sides were sealed or are in good condition, (and if it was tagged as a s"spare" one hopes it was working and put away properly to preserve it) and then all you need is the correct refrigerant, a vacuum pump, and a set of gauges to match that refrigerant. And if it needs a refrigerant that you can't legally buy, just beware that there's a thriving business in counterfeit products out there too. Some professional aid, or simply hiring a pro, may not be a totally bad idea on this.
Refrigeration is easy: Like poker, or pro basketball.<G>