The game plays really weird tricks with your screen, so attaching a debugger will be a PITA (unless I figure out how I can run a debugger on a different machine and remotely debug the game on the one computer that it manages to run in half-broken fullscreen mode). Best I can do now is poke it and see what changes.Two suggestions for you: 1, VMs are your friend. 2, decompilation technology has gotten very good. I'd "statically analyse" the binary in a disassembler/decompiler for a while first and figure out what it's doing before actually trying to run it.
I don't think source is necessarily always making things easier either --- I've had a few times where, even with open-source software, it's easier to find the right bytes in the binary to patch than to figure out where in the (huge) source that would be, and then how to build the rest of it (along with all its hairy web of dependencies) completely unmodified from the original binary.
Especially if it's a fundamentally trivial change (like a string constant somewhere, whose desired value is the same or smaller in size), and I don't expect to make any more complex changes, I'll definitely choose opening the hex editor for a few minutes (at most) over spending perhaps hours downloading a few hundred MB of source and dependencies and figuring out how the original was built and how to reproduce that.