If you have a marine electric shop in the area check with them first. One near me in Southern California replaced my old unit with the original separate voltage regulator with an updated version with an internal regulator. They knew just what to give me for an A-4. Later, after it was damaged during an engine overhaul, they repaired at a reasonable cost. Yours may not need much work if you are lucky. If you do not have any local experts and feel you want a new one, one source is Moyer Marine. Don lists a 55 amp alternator that should do the job.