Like you, I am only a DIY-er when it comes to fibreglass. However I have rarely had any failures, and it seems your plan for repair of your centreboard is sound. Ensuring that the new 'bit' contains a similar weight of lead and has a profile slim enough to fit through the casing is basic stuff and should pose no problems.
The only tip I would offer is that when making repairs where cosmetics are concerned, I use triaxial fibreglass tape rather than chopped strand mat. The tape is enormously strong, 'wets' easily, does not distort during 'wetting out', and doesn't leave sharp and dangerous peaks to catch one's skin on. Triaxial is used where strength is crucial such as deck-to-hull joins.
The only other precaution I take when 'glassing' structural forms, is that I mix equal parts of fairing powder and glue powder when preparing the fairing mix. It remains reasonably easy to work when set, yet it provides substantial further strengthening to the repair. Here in the equatorial climes, I tend to use a slow curing part 'B' in the epoxy mix to give me a little more working time with the job.
"if at first you don't succeed....Redefine success"!