To get back to the point I did not make as well as I could have.
From the factory's point of view they both have to make a crankcase which has to be cast and machines,
They both have to make a crankshaft to take 2 con rods
They both have to cast a pair of barels out of alloy and insert an iron linner into each barrel which must be bored to a precise size
The both have to make a camshaft
They both have to make an oil pump
They both have to cast heads , fit valve seats, valve guides valves , springs & rockers.
So from a manufacturers perspective their is very little cost diffrence between a mower engine and a motorcycle engine.
In the early days a lot of motorcycle engine makers sold a slightly modified version of their motorcycle engine as a stationary engine
Same tooling same precision, detachable barrels add a small extra cost to your HD but not that much extra cost.
I would seriously doubt if the cost difference in manufacture of the two was much more than around 10% ( gearbox & clutch excluded )
However the mower company would be running at a gross profit margin in the teens while the motorcycle engine sells for so much the gross profit would be around 50%-75% mark.
And ths is because Joe Public will happily pay $ 10,000 for and engine for their bikes while they will quib at paying $ 1,000 for their mower engines,
So a small engine maker can not survive thus the lack of makers.