funny you should say that, my Portal project (which I consider to be some good mapping, tho that
could be biased
) started out as a map for the portal-gun mod in GMOD. Back then, I didn't know what Snap-To-Grid was, nor how to seal a map proper. My whole mapping ethic SUCKED. Basically, my HDD crashed so I started anew for the real Portal. I had mapped a little more for Source, and gotten way better. Recently, I went and looked at a backup of one test-chamber from my old map. I felt so ashamed that I created this misaligned, fucked-up pile of rubbish!
Oh yea, point of that ramble is this: If the core idea is good, keep it and run with it. I didn't use any of my original content, but I
did use my "blueprints" and plans for levels. 90% of them turned out to work great, needing only minor tweaking.