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You need an overrun mechanism, the drill engages to turn the engine and once it starts the coupling is automatically disconnected, think vintage car hand cranks.
After seeing Gils replacement crankshaft, i would advise not using a drill to start the M91. The high torque of a drill can cause the 5 piece crankshaft to loosen where the shaft is pressed together, causing continual timing changes and other problems. I learned this the hard way, and started making rope start pulleys for any engines that don't have a 1 piece crankshaft.crazydoug
There are a number of ebay and Amazon vendors in China selling pulleys that would work well if the 3-6 week leadtime doesn't bother you. At under $6.00 USD each, I have just ordered two...one for the M90 and a spare for the M91. Only critical measurement is the 8 mm bore.Rog