the only way to tell if the engine is trashed is to run it if it has the same amount of power as before it is not trashed otherwise it is trashed if it is is trashed you will need a new piston and cylinder liner which are cheaper than a new engine and easy to change yourself fuel is fuel so unless you ran soft scrub tub and tile cleaner through the engine instead of fuel one tank should not cause the engine to wear out it may be that your engine is getting broken in then again you may have had a lean run you said you had to let it cool down was it really hot if it was really hot then you had a lean run even though you said you never ran it lean that doesnt mean that it didnt go lean in the air did it start to lose power toward the end of the flight if so this is also a sign that it was lean anyway good luck with your engine