Without knowing more about your engine's history and condition, that sounds like a reasonable preparation for starting it for the first time. Running the engine without mounting it is a marginal thing to do in terms of safety: when you pull the starter the engine could easily tip over, and amazingly complicated things can then happen in the heat of the moment. The safe way to run an engine is with it bolted down somehow. One solution would be to remove the engine-side part of the mower's clutch, so you could mount the engine on the mower deck without the output shaft being coupled, but at this point it would be preferable to leave the mower alone and just get the engine working. Can you bolt the engine down to a box or something?
