After looking on HPI's site, my best judgment tells me that they are not compatible.

Dollar Hobbyz, on eBay, has the TF-40 (transmitter only) for about $27. For a couple bucks more, maybe a whole new radio set would interest you. For the past few years, my two FlySky FS-GT3C radios have performed great. What's really sad is, and I must confess, my third one was a complete dud after a short time of gentle use. They can be found for around $40 shipped. The FlySky radios feel kinda cheapish and are hit or miss at times. For a step up in performance and features, a $52 (if you look hard enough) RadioLink RC4GS transmitter gets my vote for a great low budget setup.