to the hardware store we will go
take the fan & heatsink with you and get some 1.5 - 2.5" long sheet metal screws, preferably aluminum or stainless steel. Just be sure they fit thru the mounting holes in the fan and are a LITTLE snug fitting into the tines of the HS, and reach almost down to but not touching the base of the HS.
DO NOT make the mistake of overtightening them or you WILL fubar your HS bigtime !!!!!!!!!!!
IIRC, I used a 40mm square low-profile fan w/ a 4pin molex connector on the G4/550 that was in my B&W. Got it from Bust&Buy for ~$4.00
