I don't know where to find the engine, but I can tell you a quick way
to tell which way to turn the nut to loosen it:
carefully pull the start cord on the engine and note which way the
shaft turns (may need a helper so one can watch and the other person
can pull the cord). The nut/shaft will be threaded so that the shaft
rotation tightens the nut (otherwise the lawnmower would throw the
blades off every time you started it up).