Hi Peter,
I might be able to help, but it will take me a while to confirm things.
I'm in temporary accommodation at the moment and everything from furniture to boat bits is in boxes piled high in the garage, so I can't get at anything until the move into our new permanent home - hopefully in a few weeks time.
However, here is a photo of my original mast step taken shortly after I bought the boat in 2004 before it was replaced with a tabernacle:
It comes from my boat, #267. I believe that yours is earlier than #154 and probably earlier than that at I believe at least some of your rigging is of the original black anodised type seen in the early brochures. I don't know if your mast was supplied by the same manufacturer or whether it will fit your mast.
If your's looks similar then do post a few measurements and when I unpack after the next house move I'll check if mine is a match. I'll never re-fit the original so it would be good to see it put to good use.
Do get your son to keep the mould when he builds your door. Spare parts for SeaHawks are sometimes requested, and it would be useful to know where one could be obtained.