If we had the source code, heck we'd ruin the game.

Everybody would try to make it better, and better, and better, and better, and.. you get it. It would be changed so much that it isn't JJ2 anymore.

Apart from that, even if the above wouldn't happen, JJ2 cannot be made perfect.

Think about the buttstomp for example. I think I've heard someone saying he would like to remove the "waiting sequence" when buttstomping (you stay in the air for about one second). That would be extremely useful for multiplayer, but in singleplayer it would make beating Bilsy a heck of a job. I know it's possible to beat Bilsy without stomping, but it's very tricky.

Think about it, I'm sure there are more of similiar things like the above.