I will kinda piggy back on what Matthew said but will speak to current day designations for military aircraft just in case you would want to fly it as such.

Military aircraft call signs are similar to those of civilian aircraft but differ in that they are <service branch+tail number> ex. R54321 or Army 54321; however, many military aircraft typically use a tactical call sign ex. TETON81 or FIREBIRD 64. In my experience, both as a controller and as a part of a ground unit with air support, this lends itself to simplifying communication.

Something along the lines of MUSTANG51 would work or, like Matthew suggested, research some of the era-specific tactical call signs used.