Noun for something that reverse engineers something else?

A reverse-engineered piece of software: something developed using reverse-engineering approach to another product.

A recent practical example: Hamrick Software company developed (legally, with a permission from authors) a 64-bit driver version for an older Epson scanner.

We reverse engineered the Epson Perfection 3200 driver and included it in VueScan so you can keep using your old scanner. https://www.hamrick.com/vuescan/epson_perfection_3200.html

So according to their own words, they included a 'reverse-engineered Epson driver' in their scan software. In your example:

Application X is reverse-engineered Y.

Phillip K. Dick in his short story Paycheck illustrated your quandary of the vocation of a Reverse Engineer succinctly but never coined a unique term for it. As technology’s complexity makes this job description a profession I would venture to coin a new term that of ‘Revgineer’ I’m just spit-balling here but it may be time to step up and petition Encarta/Webster’s/Britannica etc. for acceptance of a ‘New” term.