Unfortunitely, you may have damaged more than just the piston and sleeve (scratch), closely inspect the wristpin, conrod and/or crank.
I'd try upping the nitro content to 20 or 30% before rebuilding.
The higher nitro content has a cooling effect when run just a tad rich, perfect for extending bashing time. You'll need to reset the carb to factory (rich) settings and do another "break in" procedure, it may
prolong the need for a rebuild. A nitro engine will overheat and run like pooh once its exceeded its life, its possible you got all your wear at one time, a new engine REQUIRES proper break as to seat the piston to the sleeve, much like rings in a gas engine.

RTR means "ready to rebuild" for most manufacturers and to eliminate any surprises you should get familiar with your RC's inner workings before installing the batteries or fueling up.

Something you said doesn't make sence, with the radio/receiver on, the servo wouldn't go WOT, regardless of polarity (hooked up backwards) unless your applying brakes or throttle input, or you failed to set the end points (if applicable). Under normal conditions the servo goes to the neutral (middle) position when energized with the TX on. Even if the linkage needed to be tweeked, surely it wasn't to the point that the servo was neutral yet at WOT. It sounds as if you didn't have both the TX and RX on and/or didn't have the radio adjusted properly.
Come on, FESS UP!