It sounds like you need to add a considerable amount of weight. I would recommend adding weight that would be functional instead of just dead weight. Is it a problem for the vessel to be lighter for stability reasons? I would rather add either additional fuel tanks, water tanks, waste oil tank, clean oil tank, or something that could hold something useful instead of loading the vessel with lead if it's more then 500lbs of ballast. If the old Man engines are still available, take one of them in a truck to a weighing place and weigh the truck with and without the engine in it.