I to purchased a second hand Revanna that had the same chacteristics, the boat was a dog.
I sought advise from all and sundry to no avail, the boat was still a dog.
In desparation I moved the center of lateral resistance aft a long way, and increased the area of resistance
The thickness of my fin was 28mm it is now 18mm (current nat. champions sister boat is 13mm)
The new fin was fabricated from ply wood and fits into the original fin casing, I am on the third iteration
A larger rudder was fitted
Lead was redesigned to leave one vortex rather than two
The boat now behaves as you would expect
I am happy to supply more details if you are interested
Fair winds
David