[pschoonveld]

I second erps' guess. The easiest to do is to get a dowel or similar, pull the plug and stick it through the hole. If the piston moves back just from the pressure of the dowel, you have a broken rod or crank. If it doesn't move smoothly when pulling the rope, the same is likely true.

There are many people that would love to rebuild an old classic like this, but if you don't know how to do it yourself, you're better off to look for a new or secondhand motor.