This...

http://www.hobbypartz.com/07e-sl-427...ack-motor.html

But it is SO much larger than the stocker, I don't know if it will even fit. Now I see why you were asking for a 540 motor...the best 36mm diameter motor for 4S use is probably this one...
http://www.amainhobbies.com/product_...s-Motor-2400kV

but again I wonder if a 7cm long motor will fit, and to be honest with you, even with that (second) motor, you aren't going to be able to keep up with any comparable 1/8th car that uses 1/8th size electronics (like the first motor I linked).

You have a job ahead of you making that car trackworthy. Consider that the guys at the track have spent upwards of 700 bucks on their cars, some well over $1000.