It would be basically a plate with a gear like imprint of steel on it (partial gear welded to the plate) and set so that it engages the flywheel when bolted in place.

I've seen these before, they are actually quite easy to make, you just have to take a clay mold of the flywheel gear.

My process is to put some non-hardening modeling clay on a plate (usually a single bolt head or some such to anchor the blob of clay) and push the plate gently into the starter position. Remove it and you have your pattern for the tool. Machine the part and weld it or bolt it to the plate.

For the home builder/mechanic, I'd bolt it to the plate, just in case you need to add washers to get it offset a little differently.

Cheers!