You shouldn't be able to hold your hand on the cylinders, the temp of those can get upwards of 180 degrees. 120 would be hot.
Black coolant would indicate contamination. I would drain and flush the old coolant and refill with a good quality ethylene glycol antifreeze, such as Prestone. 50/50 works, some guys go with 60/40 if they ride in really cold weather. 50/50 will protect down to -34F, which is colder than I would want to be out sledding.
You might want to pull the recoil starter assembly and take a look at the belt, too. The belt drives the water pump and the oil pump, so if it fails, your engine doesn't like it much. It's not difficult to replace, just hard to get to; that bolt on the bottom of the recoil assembly is a pain.
Welcome to the forum!