Most of the Hobie Bobs are designed to rivet to the mast and rest on the level top. Your masthead has a sing round top and is much larger. The rounded masthead is going to be the biggest problem.
I think you're going to have to make a rig that will conform to the shape of the old style masthead. You take aluminum stock and make a jig to conform to the masthead sides and extend down the side of the mast. Rivet the aluminum jig to the mast and bolt it to the bob.
Aluminum will be easy to shape but might not be strong enough to hold the bob when it strikes the water. I would use at least 1/8" thick stock. If you're creative you can cut it in the shape of a "T" with the horizontal part as the bob mount and the vertical part as the mast connection.
I hope this makes sense. Just a thought. I'm sure someone on the forum will add their two cents.
Here's a link for you.
http://www.hobiecat.com/support/pdfs/30113.pdf
Good luck.