There isn't that much difference between a .25-size plane and a .40-size. Your TT Challenger should fly fine on most .25-size engines like the TT GP-25 or the O.S. .25 LA. You might even want to consider the new Thunder Tiger GP-28 that was recently introduced. It has some new construction features designed to make it more reliable and lower maintenence, and it's very reasonably priced:

[link=http://www.acehobby.com/ace/TTR9028.htm]Thunder Tiger GP-28 info[/link]

[link=http://www.infinitehobbies.com/moreinfo.php?pid=1990]GP-28 $69.99 @ Infinite Hobbies[/link]

Of course, your local hobby dealer is a Thunder Tiger reseller, so he should have no problem ordering the engine in for you as well. Just ask for part number TTR9028.

Good luck and good shopping!