[Dom9]

There is a bare 5513 case on ebay (from Italy) at the moment... but I would tread carefully.

Suspect the aftermarket case could be Helenarou if the shape is off but it fits the 5513 internals. If it is, the crystal ring, bezel, insert, plexi, etc. won't fit an authentic Rolex case, so you'd need all those parts as well.

As you say; may be better buying a cheap complete watch, full of service parts (to keep costs down), perhaps in the right original serial/date range and swapping the complete internals (10min job for a watchmaker, as they don't have to remove the dial, hands, etc.) if that will make you happy :)

Pics when you can and I am sure we can confirm what you have and what you need :)