I'm not sure how many links are in your chain but the chain size is a #530.
The first number is the pitch or distance from roller to roller given in eights of an inch so the 5 represents 5/8 inches.
The second number is the width of the rollers measured in eights of an inch so the 3 indicates the rollers are 3/8 inches long.
To figure out the number of links in your chain use the master link (with the removable side link and split locking retainer) as number 1 and start counting until you get to the master link again.
There are a lot of good aftermarket chains available and IMO a standard chain will work just as good as one of the expensive O-ring or X-ring chains on a Royal Enfield.