I’d definitely replace that flywheel as it is starting to crack next to the keyway. Flywheels from 18 and 20 horse motors thru the early seventies will fit although they are a different design. The same 316583 cam and 120395 key were also used on later 5.5, 6, 10, 18, and 20 horse motors so there are plenty of them out there.
Here’s a ’58 FD which I used a later flywheel on as the original one was NFG to.
