Actually the blade on a lawnmower engine doesn't act as a flywheel the flywheel acts as the flywheel, the blade is the load. It's no different then a horizontal shaft motor except it's designed to have the oil sit on one side of the case with the crank going through it.
Several high end commercial push mowers have a clutch on the blade so that when you let go of the handle the blade stops but the motor keeps running.
Something running a to PTO type set up might be different don't know. When I was in high school years back. third year students had to make model how to stuff works type projects for first year students.
What I chose was a lawn mower engine. I took one off the deck , and bolted it to a wooden pedestal for a stand that sat on the floor. Took off the valve cover made and a plexiglass window for it. I drilled holes with a hole saw around the block put windows there too. Then put a piece of plexiglass over the head a little thicker than 1 inch thick.
I used a wroughter to shape out the combustion chamber. I made three heads I stripped out the spark plug cutting threads on two of them. A retired mechanic neibor across the street did my threads for the last one. He used a bit of heat from a cigarette lighter and cut the threads very slowly.
It looked great the head blew out the cork gasket I used and eventually caught on fire. The shop teacher kept that thing for years. They would make a new head for it when they could. When the head was new it was like watching a dimmily lite strobe light. The oil splasher and the valves could be observed through the windows. It never ran right without the blade. Timing was off. That's as far as I got with it graduated at that point.