Somebody please correct me if I'm wrong but I think any 540 motor will physically fit, however if you still have the stock ESC you may be limited to 21 turn motors. Don't take my word for it though 'cos I'm not 100% sure
As it happens I've just taken a 13t limit ESC and 15t motor (used twice) out of one of my models which you can have for £15 posted.
Either that or I have a RS speed tuned 21 turn motor (used once) knocking around you can have for a fiver.