Mike I have a MKII RL24 that had a rusty keel, I removed the keel cleaned it up with a course wire brush on a 9 inch angle grinder coated it with rust reverser and layed 2 layers of fiberglass over it faired it up for a good finish and painted it with a good 2 pack paint. It has now done two seasons and still loocks like a new keel.
You need to be carefull that there is enough width in the centre case to allow for the layers of glass
rusty centre boards lose wieght ,plastic fillers do not replace this
there is an old panel beaters method of fairing called "lead wiping" use to fair car body panels which would suit this restoration
hence adding wieght to the board,this would be the ideal fix
There are plenty of options.
One system involves removing any loose flaky rust, priming with an epoxy petetrating primer such as Ameron Sea Seal 919 or Altex pre-prime 167 and then (after filling and fairing with epoxy), top coating with a hard epoxy paint such as Ameron SeaGuard 404.
That should do the trick