Before we go thinking you can put a 505 crank in a 504, you cannot. the aftermarket sachs cranks don't fit a 504. there is no "D" crank for a 504: Read this one: https://www.mopedarmy.com/forums/read.php?1,4257783
If you wanna get clever you can put a 505 crank in a 504 but you need to run a late model smaller tomos flywheel or a puch sized HPI and also make your own stator mounts and spacers and then set timing without the key. You also have to grind down the old 504 stator mounts because theres a lip so you can't just bolt on a different stator. not cheap or easy, but doable
Theres a wiki on "Sachs motor differences" with a lot of other good general details.
also, that's definitely fatigue failure - the flywheel probably got loose and with the start&stop, and hard engagement, it was wallowing out the keyway, allowing it to twist farther forward and back until you get a crack growing, then the flywheel gets enough offcenter and every single rotation at like 5000 a minute, is a little tiny flex, like bending a paperclip over and over and over til it cracks. shitty.
I don't have any, I looked yesterday and only got one 504 that I do still need.