Is it the mounting bracket that broke? Or do you need to replace the clear plastic lens? If it is the bracket, Rudy at D&R may have one, however....I suspect that the replacement light that Joe suggested will be less costly than a new bracket. That PERKO light was used on later O'DAY boats (just make sure you get the Steaming, also know as Masthead, light and not the sternlight that PERKO makes that looks almost identical.
If the clear lense is broken, again..Rudy probably has one, but I'd check a local marine store (bring old lense or the top part of the light to check fit) or perhaps DEFENDER on the web.