Firstly I'd like to thank you for answering my n00b questions.
Earlier I took the initiative to try and edit the words by myself. I was careful not to add any date only change and it corrupted the ROM. Any idea?
I guess it could be a number of things, but the first thing that comes to mind is stuff like the spaces between the words. When you put a space where there wasn't one previously make sure that the spaces that you insert are represented by the same hex values as what the original game uses as well as in other similar instances. For example, what may look like a period on the text side could be any number of different values on the hex side. You will also likely have to experiment with the control codes to document these yourself. There's no avoiding that since the game has not already been documented in this fashion.
Also, if you cut/paste some text into the ROM (which I would NOT recommend in this case) be sure that you are overwriting data as opposed to adding
data. If you overwrite data it will only change what you have overwritten, but if you add data you are likely to ruin every bit of data that lies beyond it as everything that comes after the stuff you have inserted will now be shifted to different addresses than what the game expects.
You said there is some free space, how do you know?
I just skimmed through the ROM and noticed a few places where there were large groups of "FF" bytes (empty space). Anywhere you see a few rows of nothing but "FF" that means you've got free space.
How much is there?
Not sure. I did notice a few places but I didn't bother to count bytes or anything.
How do I use it?
Put something in these empty areas that you would like to include in the game and tell the game when and how to load it. I guessing that since it's an NES game this part could get pretty complicated. I'm not sure how straight-forward the addressing is and how different mappers effect it. I've mostly just worked with the SNES.