Repair manuals for old units are not common, so you'll have to depend on logical intuition instead. Your assumption that the hi-limit is at fault is not correct...it is there merely to keep the house from burning down. In this case, the operating thermostat
controls the heat, so you would consider replacing it. Located on the blower housing, the part is shown below and you may click to order.
Other factors that induce overheating are worn drum seals and a restricted exhaust.