Depends what is cracked. If it is the OUTER case, don't worry. If it is the INNER back wall, look at it very closely. Most are covered with an enamel coating. So, is it just a crack in the coating with solid metal behind that? Or, is the actual metal panel cracked? You NEED the metal panel to be continuous without cracks in order to prevent significant leakage of microwaves through the walls of the interior chamber.
How old is it?Feels like metal, some corrosion where the crack separates. And odd place to split wide open like that.
Just melt some lead and pour it in the crack.
You don't have to melt it. Just chew a piece of lead to soften it, then push it in the crack.