You first have to learn what a Diesel engine is about. Many re-mappers are just a bit into IT. This is not what re-mapping is about.
Anyone can alter mapping tables with the right software. Just go to Alientech or Dimsport, which are 2 of a number of suppliers. Ther you can buy the basic hard and software for about 7k and get a training course lasting 2 days.
Unfortunately this is just the beginning.
I don't know where you got the claim from that it is easy to tune rotary engines. It isn't. Bodging is easy, but not proper tuning. Not even a lot of re-mappers get things right. It is as difficult as that!