Hi down under: I raced Marbleheads for years and they are a great class of boat.
They are large enough and heavy enough that they perform much better than the currect crop of smaller plastic boats being sold.
As the other fellow stated, you can get details from AMYA. They are also referred to as the 50/500 class which means they are 50" long and have 500 square inches of sail area.
You can use any design rig you want as long as it comes in at or below the maximum sail area. I had some with wood masts, some with aluminum masts and usually did my own rigging. I still have a hull that I believe was designed and raced in Australia but can't recall the name but it has a canoe stern and sailed beautifully.

Have fun

3dbob in the mountains of North Carolina