Well, while there may be some scraps of paperwork with Senna's name on it in the history files, there are no "CoA"s for these sorts of cars. Senna's sudden death precluded any such would-be buyer comforts. Maybe you can get a notarised letter or similar.
I think the red one has significant provenance as there's the famous photo of Senna washing it with a garden hose(!), and if the paperwork shows that the car hasn't had a registration plate change, that's as good as you're going to get. And from the bidding interest on the black one, I'd say there are some moneyed-up buyers who are also convinced on that. It's arguable, of course, how much Senna actually drove either.
(I'd rather have Senna's #11 W201 Mercedes 16V, which is in Mercedes' museum, and in which he did some genuinely important driving.)