The diameter might be larger but you could sand it down to fit into your parts. This is about the only solution I can suggest as finding a complete replacement base will be extremely difficult. If you lose or break either of those parts, you will have a stand up paddle board.
I have 26 years experience windsurfing and can answer many questions regarding technique for all levels, equipment rigging, how to go fast, some tricks, and where to sail in your area. I can also provide many links to info regarding travel, gear, and how-to`s. For those looking for older parts or general info regarding older equipment, I can usually help.
26 years windsurfing throughout many parts of the USA & Canada.