Apologies for the late reply, the last couple of days have been a little overwhelming, which is fantastic because we have so many people interested in contributing and donating to the competition.
As far as defining what we consider a port, and what's acceptable for the contest, a "port" can include the use of original source code and assets, as well as ground-up recreations. Provided it's recognisable as the game it's based on, it should be accepted. If it's a port of a game that is currently for sale, it shouldn't include any assets that haven't been released for free (for example a Doom port requires the WAD file from a registered copy of the game). If the game is considered "abandonware" it's generally accepted that any recreation using the original assets will go unnoticed and unpunished. As far as I'm aware, the only game that ever receives legal consequences are ports of Mario, so avoid any games from that company. As far as I know, the consequences are that the game is taken down from the internet, nothing worse. In any case, it's up to you to use your judgement on what's right or wrong.
If it's a port of a game from an inferior system, you are welcome to improve it. If you improve it to the point where it's no longer recognisable as the original game, it may not meet everyone's standards. The winners are the ones who impress the community and the judges the most, and we're all capable of considering the amount of effort and good decisions that are put into each entry. Turbo Sprint is a perfect example of a game that is clearly recognisable as a port of the original game without stepping on too many toes. and I'm sure would be voted favourably.
Remember that as a port, it's not really the kind of thing you'd be able to resell (but you could win some fantastic prizes). If the game you submit is so far removed from the original that it has become its own game, you may not win this contest but you would probably be able to sell the game.