Susan - As the others suggest, work the tractor HARD for 10 - 15 minutes (run it wide open and with as much load as you get manage). Then allow it idle no longer than a minute or so before shutting down. This will burn out every trace of moisture and gunk from and leave the manifold/muffler dry inside.
Now presuming there is no coolant leak in the head your next startup will be "clean".
ALSO if your tractor has an inoperative thermostat - or if the t'stat is missing - install a new one so the engine operates at the correct temperature.