Blown head gasket

I have a blown head gasket in my car. I have put Blue Devil in it and I did or change there was a crack in my reservoir. I fix the crack in my reservoir with a sealant. is it going to hold and if not why? are their any other problems that can cause water to get in the oil other than my blown head gasket? if so what caused it to leak? what all should I check after applying thermostat gasket? is the car worth saving as long as it still cranks and does not smoke out the tailpipe?