I have recently been doing some towing behind a J-3 cub with a 90 hp.
motor. Works great and he can fly about 40 knots. He does have a
little concern about overheating but we haven't had any trouble. He
said he could only climb about 1500' pulling a Ka-7 before temps got
too high.

My experience flying a Cessna 140 is that it wouldn't be able to fly
any slower than more common towplanes.