oh dude yea. very real common issue. I think maybe it's cuz of fun with coasterbrakes? but yours is 504 no? so I dunno.
Anyway happens alot.
this writeup pretty well covers what you need to do (i think, havent read it in a while)
timin is a little trickier but if you make a TDC mark for flywheel and case (i think there already is one) and make sure when you're putting it on that that still lines up, you should be fine.