overheating - pull the engine - pull off the fan shrould and check for stuff clogging the cover (which directs the air flow over the heads) - critters could have built nests in there while you had it parked.
Gas mileage - check the float level in the carb - and check for gas in the oil (the membrane in the fuel pump could have a tear - which would dump some gas into the crankcase) -
Best of luck -
Steve
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