Powerslave EX uses a custom engine built from scratch by the author, level designer, Samuel ‘Kaiser’ Villarreal. Recently, a public beta versions of the project was released:
https://powerslaveex.wordpress.com/2015 ... -released/
Here's a gameplay video: YouTube link.
People who have played the authentic PlayStation version of the game say that the recreation is pretty true to the original.
An interview with the project author:
http://techraptor.net/content/interview ... powerslave
As for the legal status of the project, the FAQ says the folllowing:
The engine is planned to have an open source release in the future.
- Is this legal? Who owns the rights to Powerslave?
- While I know who owns the rights, I am not going to disclose the name of that individual. That said, I have no intention of making any profit from Powerslave EX. The whole purpose of this project is to keep the spirit of the game alive and to introduce it to a whole new generation of gamers and act as a teaching tool to those who want to be a game programmer. This project will, and always will be, free. I will continue developing this project unless I am given a cease and desist letter.