It's a slightly modified version of the Duke engine. Renaming the resources(?) doesn't work as the source is different. I suppose you could make a total conversion for Duke3D, but thats also a lot of work.
I still play it from time to time, but I still think Rise of the Triad is still better (just my opinion). Below is the source from the official site (they have a lot of sources there for other games as well, btw)
ftp://ftp.3drealms.com/source/shadowwarriorsource.zip
http://www.3drealms.com/downloads.html
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