-Remove spark plug
-Insert around 2' of 1/4"-5/16" nylon rope into spark plug hole; this will prevent the engine from reaching TDC (Top Dead Centre), preventing it from turning while you tighten the bolt...
Another idea is to engage the starter while tightening the bolt but, that only works for as long as you can hold the starter rope engaged; sooner or later, the torque applied to the bolt will be greater than the tension placed on the starter rope...
...Used in conjunction with