I just ordered a Sport Werks .21 for my X-Ray XB-8. It makes good power and is a high-revving engine. Another engine that I've heard good things about is the OFNA Hyper .21. Both of these engines are around the $165 MARK.
The Swift is discontinued as far as I know. Slightly older technology, not a bad car at one time. Have a look at the CEN Matrix,very tough, easy to tune and a proven car.