briggs 450 series?

its a modern day 3 1/2 hp classic engine with pretty plastic bits . grounded heavy smashes the powerdrive pully blade spacer . i have however replaced the damaged bits .but the machine is hyraulicing when you pull the cord ( a term i am aware of as a vehicle fitter ) it rips the cord from your fingers as it atempts to fire . oil level is ok , i suspect a timing issue possibly as i am aware they have aluminium woodrift keys intended to break in the event of a ground strike ? with the plug out it turns freely , i have checked the 2 i am aware of flywheel and the steel key on the main shaft . excuse the random order of events just putting all the facts i can remember