DJM will build you a Ka Cosworth for £70,000, but that's including chassis strengthening, spaceframing and stuff like that - basically a replica of their own race cars. I tried contacting Dave ( owner ) about fitting the engine and runing gear into a Ka for me, but I got one email back from him before he stopped replying!
I've been thinking of building a Ka Cosworth for quite some time now, I have everything specced and detailed ready to go, it's just finding the fund's to do it, but I sure as hell wouldn't pay £70,000 for someone else to do it, I'd rather try it myself with my mates for a fraction of said cost.