So it might not be possible to find the source already - just contact the developers, point out that if they release the source and not the data files they'll sell more copies once a port is made (even more if they make the port themselves and sell a ready-compiled GP32 version, possibly via JoyGP) and see what they say. Considering the software is old-ish, they might be receptive, if the data isn't included within the executable (i.e. you need the data in order to compile the thing you run, as opposed to just needing the files there to actually run it).
Of course, they could say no, at which point you learn C++ and start your own remake of it (as opposed to port).