What program should I use? I can't seem to find a good one
If you're that determined to work on it then please consult someone who's well-versed in finding loopholes in such licenses. Don't take the advice of random people on this site.
Anyways, so here's the thing: the game is in Java. Even though Java is my primary language, I'd say that Java is
kinda hell when compared to other languages.
Modifying the files to get the game working or updating would be a BIG hassles, and from what I've heard, Innoxia is a terrible programmer so I don't expect the code to be decipherable as well.
If you want to make the game from scratch...ehh...good luck with that. It'll take quite some time. You need to read all the even tries and stuff. Plus you'll have to live up to the expectations of everyone.
Why don't you try modding Degrees of Lewdity? Or work on vanilla DoL to help Vrelnir. The code for it is pretty well defined, and it's an HTML game so it's far easier to work on.