fast&future, kido good luck with the project and I am proud to know that a pakistani kid like you is engaged in projects like this. Wish there were more kids like you especially in Pakistan.
go for a small lawn mawer engine. I am not sure whether you can get some lawn mawers in Karachi. Is there a scrap yard in Karachi of used car parts? maybe you might find something there.
let me know if you are coming to Islamabad, I will be more then happy to show you my project. its not a go-kart but an offroad buggy which I made myself.
It is similiar to your project however it is much bigger and with a real engine in it.
good luck & keep up the good work