Boats over 20 feet long are NOT required by coast guard regulations (or here in PA, by state regulations) to have a capacity plate. HOWEVER, Coast Guard usually does certify a manufacturers design for a horsepower rating.
Although without seeing the hull, from the info you gave, I would be comfortable with a 200 HP engine, given the transom is sound and the hull is also in seaworthy condition. BEST BET is to contact the manufacturer if still in business and ask what it is rated for. OR go to a brand users site and see if you can dig up a catalog. They usually have a recommended horsepower.
For example: My Manatee is 20 1/2 feet long with a 20 inch transom and a 7 foot beam. The manufacturer's catalog actually shows it with a 90 but claims it can be powered with a 180. The same hull with two feet chopped off the back also is pictured with a 90 for power but claims max horsepower would be 150.
