I have had a couple of the old A&Ms with the black belcrank. It works (sort of), just needs a little tinkering to get the idle and full throttle timing synchronized. Here is a link to the C&H ignition page with timing instructions: http://www.ch-ignitions.com/CHinst.html
Make sure you keep the plug grounded and hooked to the lead when the ignition is powered up. Don't let the spark jump to open "space".
If it has the flat surface on the front of the crank that RCIGN1 noted, it is a crank that was built by A&M. Don't let it get a prop strike as they bend very easy. If there is a dimple in the end of the crank for a machine center, then it is a reworked Sachs crank, & much stronger.
It's strong engine, and if you get set-up right, it's a sweety.