Reprogramming the whole thing will take a hella lot of time, especially if you have to learn programming from scratch. I'm not really sure I'd recommend doing it, unless you have a LOT of dedication.
Learning C from scratch is... while not the most difficult thing in the world... still very time consuming, especially if you want to write good programs (i.e. non memory-leaky, buggy, slow programs. ugh memory leaks
< I still have nightmares.) Also, if you want to program a 3d game, you most likely want a good background in trigonometry and multi-variable calculus. And linear algebra + differential equations if you actually want to mod the engine yourself, to include additional things.
In addition to programming, you'd have to learn 3d modeling and animation, something else that takes a while on its own.
If you just want to get the basics done, you can try a game maker environment. Don't be fooled by the "no programming experience necessary." They all take some time to learn as well if you want to make anything half decent. Try searching up gamemaker or 3d game maker on google. I'm not too experienced with them, but I've heard that they can be decent if you put forth some effort.
If I sound like I'm discouraging you, I... probably am
. I'd recommend going through some more school before attempting, or you might find yourself wasting a lot of time. However, if you have the drive, go for it. You'll might learn a whole lot, get what you want done, and have the rights to come back here and go HA! I WIN!, at which point I'll genuflect (and get a bad end).