In fact, you can get Fitaly 3.5 to work with WM2003. When downloading the demo, download the CAB file instead of the EXE file.
Link: http://fitaly.com/wince/pocketfitaly35cabs.zip
Copy the relevant CAB file over to your device, open File Explorer, and tap on it. This will initiate the installation process. However, you will get a pop-up questioning compatbility. Ignore it and allow it to install. Once installed, you should give your device a soft-reset. From then on, open your IME menu, and you will see Fitaly installed.