Plenty of ways to do chemistry to breathe fire, most of them involving gasses which either catch fire when two gasses mix or simply by sparks (could come from the teeth). Or flammable saliva. Endless options. It's not like we don't have weird enough animals with bioluminescence, electric eels, a million poisons, and bacteria which can heat a compost pile to spontaneous combustion, etc.

But flight is indeed unrealistic for most dragons which are supposed to be large and heavy with insufficient wingspan.