seeing that you are a serpent fan you could go for the mega .21 off road engine.Run great ,nice power band ,holds tune great and Mega quality.
There are lots of alternatives to go for just check what is popular at the local track.
casterracing EX-1 Pro, SP Silver Arrow
TOP Racing Scythe, SP 17.5, SP GT Pro 2.0
TOP Racing Scythe, SP 4.5 V2, Hobbywing Xerun 120SD 2.0
TOP Racing Scythe, SP 13.5, Hobbywing Xerun 120SD 2.0