Remove the valve cover from nonworking side. Make sure pushrods are in place as mentioned already. I think these are the Briggs engines with vanishing pushrods. People have removed valve cover and unable to find the pushrod. I think they slip inside the case halves thru a small oil drain opening.
If the pushrods are there and in the right place, then check to make sure the piston is moving up and down. I had this once and it ended up being a broken connecting rod. The engine ran but only had half power. Lost my shirt on that mower.