It is unlikely that the capacitor is the problem.
The most likely cause of charging system failure on the RV-12 is the Ducati voltage regulator.
Some RV-12 owners have replaced the Ducati with John Deere part number AM101406, or MIA881279.
If that is done, the connector will not fit and wires will have to be switched around.
Another option is to use a Silent Hektik regulator.