here is a definate, proven complete answer.
NO IT WON'T FIT.
THE FLYWHEEL FOR THE 504 IS TOO SMALL.
THE SHAFT IT MOUNTS ON IS DIFFRENT
YOU CAN'T EVEN SWAP THE STATOR PLATE AND MAKE IT FIT.
IT WON'T FIT
The stroke difference is only between a D crank and every other crank for a 505.