I am sorry to say that I cannot find any data about this ESC, and not even one for sale on EBay.
There are very few tests that can be done externally unless the casing can be removed, and these are of limited use without access to repair. Your best tool is eyes. As you do not mention anything heating up, an open circuit is a likely possibility
Its worth while carefully inspecting the wires for cracks near the casing, they break where they bend, especially the thin servo wires. Check these in the connector in case one has become separated from the crimp. One does tend to pull the wires rather than be able to grasp the connector.
Old black wires can degrade, somehow the external insulation reacts with the copper. I've seen this happen with wire stored in my workshop. The copper goes black and is practically unusable. It makes a poor connection and is difficult if not impossible to solder. So look closely at all black wires.
If it contains a processor and is very old, its not impossible that the memory has aged and become faulty depending on the storage technology. Some parts from the 90s were only specified for a ten year life.
Try a substitute and if that works then at least you have some sort of answer. But do recheck all the wiring polarities first just in case someting has gone adrift.