[b0b_d0e]

While I agree that the original source code for the game is probably long gone, I think we can settle for having a disassembled version of it. As you pointed out, the game was most likely in handwritten assembly, so the assembly output probably isn't much different. The most useful part would have been the comments, which it turns out there were some very dedicated people that went through the entire disassembly and annotated the source code like this one by doppleganger http://www.romhacking.net/documents/344/ and this one where movax tried to make the assembly more natural http://www.romhacking.net/documents/635/

There are several really cool things you can find on here about older games including a detailed explaination of the minus one world http://www.romhacking.net/documents/343/ where doppelganger covers exactly why the game does what it does.