Sorry I edited my comment because shortly afterwards I found an article about them making the original open source. Is someone charging for a Dreamcast version?
I saw this in their article that I linked above
It’s definitely been a wild ride for us all, and POSTAL means a lot to us – it’s our baby… But now we’re ready to hand the future of ‘the little shooter that could’ to the public at large. People have been asking, and we have been promising this for years now, but today we are proud to announce that the source code for POSTAL is officially released to the public on Bitbucket, under the GPL2 license. Everyone now has ‘under the hood’ access, to see what makes POSTAL tick, and anyone with the time and skills can now tweak/change/update/modify anything in the game at all! And hey, if anyone feels the urge to port the game to other platforms (The Dreamcast, for example winkwink), then they absolutely can!