Same engine that came in my Hammond new Merc 260. I've heard that a good welder can weld cast iron. There's not too much pressure at that point so a option would be to grind the rust away, clean with strong solvent, add JB weld or Allmetal. Now as to the thermal expansion and how it will handle it that's not a given. Might leak again and might not.
The new style center riser will work but all associated hardware will be required also.