What kind of metal is a dirt bike motor?

Generally motorcycle blocks are made of either cast Iron, or Cast Aluminium. A lot of custom metals are available for the highest end race teams, but odds are if you run into a modern dirt bike, the block will be cast aluminium, the pistons will be a steel alloy, the crank will be either cast iron or a steel alloy, and the Connecting rods (between the pistons and the crank) will be either a steel alloy, or an aluminium alloy. Possibly titanium in various percentages in any or all components.
Older engines will be generally the same, but the block will be more than likely made of cast iron.