Windows CE 7 is a really odd "legacy system", so consider that it got
killed someday!!!
To prepare for that pay attentation to "clean code". I mean that you separate portable and platform specific code. I normally use clean C++ code for that. Often helps a sound software architecture to recode not portable items. Use clean interfaces like SQL, csv or XML. Maybe some web services will help you sending data to a web server. (Security!!!) It has the advantage, that these services are available from every platfrom.
Modern development platforms are the actual Visual Studio (2015) or Xamarin which are good for portable solution, but I guess they wont support your WinCE. But you can try them out, and most important check your portable code for compilation and some errors.
My tip: Discuss with client some migration strategy for
web services. It may go to Windows Mobile (10), Android or iOS. Important is how long the Casio devices will get sold and support like spare parts.