The fin at the back of the cockpit will work apparently, have seen it done and with no hatches on that boat it'll be much easier to access that location.

Personally with an old down river kayak I would just use the boat and not worry about it's cosmetics, it's not the right kayak for flat water regardless of how it could be painted. Not a lot of reason to sink a bunch of money into it, IMHO.

For what it'll cost to make it pretty, you could easily build a new SOF kayak that is actually designed for flat water use.

Bill H.