I would ask your transom guy what you can do. BUT if its a 23 I/O definitely approach it from the inside. You can make 2 small cuts across the hawspipes and remove the rear portion of the cap. It leaves plenty of room to work. Then I would remove the inner skin, core, and grind down the outer skin (prep it). That way you guy can just do the layup. OR you can just get a bunch of beer, resin and glass and do it yourself
If you search old posts its been done before. They are what helped me.