What standard roller chain size is the same as motorcycle roller chain size 520?

There really isn't one...

50 Series chain is a 5/8" pitch with a .400" dia roller that is 3/8" wide. I directly equates to a motorcycle 530 chain in size.

520 - 5/8" pitch .400 dia roller 1/4" width.

525 - 5/8" pitch .400 dia roller 5/16" width.

530 - 5/8" pitch .400 dia roller 3/8" width.