typically when you edit something like that in a game it is just a code that has to be changed. usually with stuff like that you can look through the folders of the game install location and do a couple of "ctrl F"s until you find a keyword close to what you were looking for. other times a simple google search for "change this" in "this game" can lead you to the route your looking for, and isnt always a bad place to start, specially with already heavily modded games.
As far as access to the game engine it self... well, thats not so easy. most of the engines used in games today have been tortured over the last few years to get them where they are now, and those that shed the blood sweat and tears are never likely to hand it over without a very, very steep price. and good credibility in the industry from one fashion to another. however, if your looking into doing some simple game ideas with just a rough setup and see if you like it you can get unity and blender for free.Unity
is a free to use engine that has tons of tutorials strewn across the web, its also very user friendly and not too resource heavy(depending on how much you throw at it)Blender
is another fun and easy to learn program that lets you create 3d objects and upload them into Unity as characters weapons obstacle etc, again lots of tutorials on script and modeling all over the web.
both of them are free to use and have a lot of online support. if your looking at this for a new hobbie and wouldnt mind spending, say a new games worth on it, you can check out cgcookie
5 months expert no. but i spent about $60 at cg cookie and about 6 months of my deployment to afghanistan playing with blender and i have to say, its not as hard as i thought it would be and the tutorials online are great, really cool when you see a design come togetherEdited by microfister - 3/7/12 at 4:23am