You Can leave the head in place according to the WIS (21 pages worth of info on doing the guide rails..). I would get new seals for the timing cover cam.
I'm surprised a new rail let go on you so soon:-/
IMHO, this is a rather rare occurance you have come across.. So don't get to down on it just yet..I would however ditch your 203..owning any new MB w/o a warantee is sketchy at best!
Here's a link to the image as the limits here are quite low:
http://jono.mbfans.net/190Rail.jpg
Jonathan