I don't know of any one who had done this--if the cabin dimensions are similar to the modern Angler or Cruiser, then the factory should be able to sell you a modern cabin top. They are set on top, and then just tabbed in with glass tape. Not a really hard job to do, cut the old tape off with a Fein Saw. Set the new one in place--tack down the 4 corners with a quick setting adhesive, then tab in the new glass cloth.
To try and make the new top could be done, starting with door skins as the inside mold, and build glass on it I would use hot glue for first mock up with cardboard--either in a model or full size. (Probably both) Then use "stitch and glue, to make the first set up for the interior plywood. Drill holes about 1/4" back from the edge of the properly cut pieces of door skins, use solid copper wire thru the holes, to hold the wood in place. Use 3" wide glass tape and epoxy to "glue between the copper wires. After the epoxy has set up, remove the wires, lay a fillet in side and outside corners--and then start laying up several layers of 1708 glass and polyester resin. Finish off by fairing and gel coat. Doing this you would use your old top, put the "dome" on it. and fair inside and outside into the original. Quite a bit of work. No idea what C Dory would charge to make you one of the current types.