In what plane, towing what size banners, in what ambient temperatures? Most likely the cylinders are toast, unless it was operated in an "open cowl" Cub. In that case, it's probably no worse than any other engine, as the cylinders run at reasonable temperature in that configuration. In the dark ages, (aka good old days) we ran extra oil coolers and kept the oil cold, but were blissfuly ignorant of the fact that the cylinders were being fried. It's all about the particular installation, the operation, and how the cooling was modified - or not. When I asked my dear old dad about a CHT gauge, he replied snorting... What are you going to do if it's too hot? turn around and go back??