Fix or write a new mobile app (iOS and Android)

It´s a mental training app (https://apps.apple.com/se/app/mental-tr%C3%A4ning/id1184875849), that uses Ecwid (https://www.ecwid.com/) as backend for buying courses as in-app purchases. All present code is in the zip -file and also a PDF of present development status. I guess there code (Xamarin i C#), that is 5 years old, is outdated and better to rewrite from scratch.