Most Yanmars have a 35amp Hitachi as standard. At least every one I have seen. Don't know if I would go any bigger on a one lunger. Make a trip to your local alternator/starter shop. Save yourself a bunch of bucks. Recently told by my gf that she bought a 80 amp alternator from Yanmar, for nearly 600 bucks. I just upgraded mine to a 70 amp from a local rebuilder. Brand new, 70amp Hitachi, for 80 bucks. Don't get drug into the marine alternator deal. The difference in a marine alternator and a non marine, are little screens inside that stop sparks from escaping. These are intended to keep a hot spark from igniting gas fumes. Not needed on diesel motors.
Dream like you will live forever. Live like you will die tomorrow. ( James Dean )