I'm not sure this qualifies...
My gramps identified some problems in small 4hp 4 stroke Yamaha outboard engines.
If memory serves me right, it was hot start issues, stuttering with sudden throttle changes after long periods of constant throttle, and ease of cleaning the fuel filter at sea without unhooking...
Not stuff that would affect a casual user much, but a nuisance for the many pros that used these (tax exempt) engines nonstop, 8 hours a day, for a million uses in ports, ship yards, fishing etc.
Being a mechanical engineer, he managed to narrow the modification to a few parts with minimal labour.
He took his engine to the regional distributor to test, and soon was modifying every engine of that type sold in that area.
He did it himself, because he enjoyed it, but could have outsourced the work.
It paid well for about 5 years until the next generation of engines had the issues solved by the manufacturer.