I thought I had seen somewhere that somebody had a two-motor upgrade/replacement tail motor setup.
This is not the best option if you want a better tail. The Direct Drive system is the best way to go. GWS sells a tail kit that is for a DD setup. The DragonFly II DD model, uses this setup and will fit directly on the CP. You will need a DD motor, GWS tail/vertical fin, and DD prop. There is much info about this DD setup on this thread and on other sites on the web.