That one that Tim posted would need access to the flywheel ring gear teeth. Not the most convenient way to hand turn an engine.
On a stovebolt engine that had a crank, take a cheapo 1/2" extension and drill a 1/4" hole thru the square end and drive a piece of 1/4" steel rod thru so it's sticking out about 1/4" or so on either side. The 1/4" rod will engage the balancer where the original crank did. You may have to tack weld the rod in place on the end of the extension. You need a bit of the extension sticking out the end to center it in the balancer. I was going to take a picture of the end of my original crank, but it's buried behind a bunch of other stuff in the garage.
If the spark plugs are out, you should be able to turn the engine with the fan blades as long as the belt is snug.
