If it WERE/WAS mine...

In all indicative mood sentences, it is a singular pronoun taking the singular verb.  The example that you have exhumed, however, is a use of the subjunctive were used in conditional sentences stating a present condition contrary to fact (the trophy is not in fact the speaker's).  Were appears with all subjects in this type of sentence:

If it were raining now, I would be very wet.
If I were king, I would rid the nation of its woes.
If she were cuter, I would ask her for a date.
If they were rich, they wouldn't be borrowing my car.
etc.

This use of the subjunctive is quite formal these days, and you will often see was in the place of were with1st and 3rd person singular subjects.

Mister Micawber wrote:

In all indicative mood sentences, it is a singular pronoun taking the singular verb.  The example that you have exhumed, however, is a use of the subjunctive were used in conditional sentences stating a present condition contrary to fact (the trophy is not in fact the speaker's).  Were appears with all subjects in this type of sentence:

Unfortunately, there are those well-meaning, educated folk who misuse the subjunctive after "if", when a simple indicative is needed:  "I did't know if he were coming or not" is not an example of contrary-to-fact and should be "I didn't know if he was coming or not".  This is quite different from "If he were coming I would bake a cake" (but we know that he isn't coming).