I have never written a mutator before, but I would be willing to learn. I have all the files for the "GoKart" (This is not my creation, all credit for the vehicle itself goes to it's creator), but I don't know how I would make it into a vehicle replacement mutator. For example, the GoKart mutator would replace all scorpions on the map with the "GoKart".
Of the vehicle? It is found on spacecargobeta3, and it shoots green and red shells from mario kart. Pretty sure it originally came in the Onslaught toys #1 pack. I hope this helps