Can you change a 4 stroke to a 2 stroke dirt bike?

With enough effort, yes. But it'll probably be better to simply sell the 4-stroke and buy a 2-stroke instead.

2-strokes rev higher, but have less torque, so you'll probably want to use a gearbox intended for a 2-stroke as well.

And don't expect engine mounts and stuff simply to line up, you may well have to have new brackets made and added tot he frame.