I would be hesitant to clip a 120mm fan onto the heat sink - might put extra twist or stress on the heat sink clips or CPU mount.
Instead you might consider how to mount the fan nearby so it provides airflow over the heat sink. You could use something like a Zalman FB123 to mount a larger fan to blow on the heatsink without attaching the fan to the heatsink.
The posting below is an example of where I adapted an 80mm case fan bracket to hold an 80mm fan to cool CPU in a slot-1 system.
http://www.silentpcreview.com/forums/vi ... highlight=
There are some slot-1 heat sinks that were designed to work without fans (larger) - I have a couple that came on 600 MHz or so CPUs, not sure how such would do cooling 1GHz cpus.
Most of the slot-1 coolers I have seen have a thermal transfer pad. Think they were designed for one-time application, so if reusing the heatsink may need to remove the old pad and apply thermal compound.
(May also find that using heatsink compound gives lower temperature than using the original pad.)