its an old johnson motor flywheel that op is asking how to hold to get it tight. as i posted a screw driver in the flywheel teeth locked against the starter will hold it whilst the 100 foot pound is on the nut whilst it is tightened.
forget the rattle gun it will trash the motor , the wife will run off with the gardener and the dog will crap on the lawn , so best go by 200 buck worth of tools so your life does not end because of a nut that is maybe 10 foot pound either way of correct torque on a1980s motor.
,i just sent about 10 old johno flywheels to scrap from dead motors non of which died from over tightening the flywheel.
you guys over think a bit when it comes to slapping old motors together, these things can withstand backyard mechanics fixing them for years because they are as solid as a rock.
now that said , when i am working on a lets say a 2018 300 + horse yammy or suki motor, then i use a torque wrench on almost every bolt or screw, because that is what is expected from me to ensure the motors are up to """my standard "" when reassembled , also the newer motors are no where near as resilient as the good old beasts the OP has.