Wouldn't it simply be easier to persuade the developers to alter the game so unit values are loaded from a resource file?

Going to the extent of writing a patch for an executable which you have no documentation - source code or otherwise - for seems a little drastic. I'm a programmer myself and I have doubts about whether it could be done.

Editing unit attributes is ok, since this is likely to be held in a data section and would not require modification of any executable code. But it is possible, even likely, that the incompatibilities this would lead to would be an absolute nightmare.

Unofficial patches have a rather bad reputation and deservedly so. I would certainly never use an unofficial patch on any game of mine. You say .cpp files would be distributed, but how will non-programmers understand them? Presumably an executable version will be distributed with the source file, since very few people have compilers. What is the guarantee that the executable and the source file match?

Do I misunderstand what you are trying to do?