Word of advice -- put the full game name in a least once, or make it obvious what you are talking about. I know it means mario kart DS but not everybody would.
The trouble you will face is modifying the download play file will break the RSA signing and we don't have the key. If your DS/DS lite has a flashme modded firmware then you can still load it but not so many people do that any more (it is easy enough and you need a flash cart for a few minutes to do it, as well as something to short the write pads for the firmware chip).
If you have that though you likely have a flash cart and that opens up so many more options.