After several original Ultimaker extruder broke, we decided to do a mod of our own. It uses the original spring, the little bearing with its M3 bolt and nut that hold the bearing, and also the M3 bolt and nut that attach the extruder to the feeder mechanism from the original Ultimaker extruder.
Updated version of block part to make it fit better
See http://www.3drucken.ch/2013/10/ultimaker-1-extruder-m4-guggelmod.html for more details.
Reused parts from original Ultimaker extruder:
1x M3 bolt and nut that hold the spring
1x M3 bolt and nut that attach the extruder to the feeder mechanism
print 1x block ultimaker 1 extruder m4 gueggelmod (use upright position and touching buildplate support, 40% infill)
print 1x slider ultimaker 1 extruder m4 gueggelmod (40% infill)
1x M4 bolt 50mm (32-40mm should be enough)
3x M4 nut (you can save 2 if you use a shorter bolt)
1x bearing 12x8x3.5mm metal