If the engine you want to reverse has a one part crankcase, you'll have to get a special crankshaft as mentioned by C. Watkins but if it has a removable front housing, all you have to do is to turn the front housing 90 degres and il will turn backward.

On engine with a rotary disk like the old OS 77 you can just switch the slot where the crank connects in the disk and it will turn backward.