Sorry to break it up to you but that would be totally pointless as theres lots of good racing games out there for the DS such as: Mario Kart, GRID, Need For Speed Series, Asphalt: Urban GT 2 and much more. Another note, racing games arent really good for the DS IMO. I know you meant homebrew but I really doubt no one would make a homebrew racing unless they have confidence to make it REALLY good.
I think a GT quality racing game (dozens of licensed vehicles, real world locations, relatively cutting-edge graphics etc) would be a little ambitious for a homebrew release. Try requesting something a little more likely such as Pole Position or Monaco GP.