Sorry, I don't have any source code for the Vortex. I did not work on that project but there might be a memory space limitation on the main 21xx chip that runs the show. You'll want to look at the data sheet for the chip to be sure before spending any more time on the project.
For code, you could just disassemble the contents of the ROM. I'm pretty sure it was all assembly language so it might be readable with the right IDE. That will at least get you the right mechanics of the code (without any names). Of course, you'll also have to get hold of the build tools (assembler/linker), a ROM burner, a ROM eraser, a ROM Emulator or some kind of embedded debugger. I'm not aware of JTAG debuggers for the 21xx chips but it's worth getting if available.
To me, this seems like a big project but you will certainly learn a lot in the process.