>>How hard would that be to get into blitz?<<
What, a wad importer, or the entire game engine?
you can find the originally in 1997 released linux source of doom 1 here:
http://www.classicdosgames.com/source.html, including the original, pretty funny readme.txt by john carmack.
I think the original doom is pretty easy to port to almost any language. It's a pretty old C++ source, I guess you won't have problems with object oriented structures. The wngine was written clearly with portability in mind, and it is using software rendereing only, so it would run in Blitz2D.
Some people from the menuetos community ported it to the menuetos OS straight forward, took them not a very long time, tho they converted it to ansi C. Not sure if it's worth the time in Blitz.
You may also create Q3 bsps using "Gmax" with the "tempest gamepack":
http://www.discreet.com/products/gmax/tho I don't know if they internally use IDs compiler too. At least this converter path:
http://hazzard21.tripod.com/convert02.html is using the original compiler.