I would have thought that a thorough search on Pprune would throw up Caravan conversion courses. The problem surely will be with the BN2. There used to be one or two operated by a coastal airline years ago but these days I suspect that only the SA military have any. That's probably the same as in Mauritius. It's operationally quite expensive and rather specific in design.
Here's a link to a register service. It might be easiest to find out who in SA even has an Islander and then approach them.
South African aircraft register
Here's another not so amusing link. Perhaps it was the last Islander?
Photos: Britten-Norman BN-2B-27 Islander Aircraft Pictures | Airliners.net