Not sure why somebody would remove the heat sink. I'm sure that amp baked itself to death rather quickly.

For the heat sink - full AAVID part number is 531102B00000G.

The B00000G version relies on the leads of the device (amplifier in this case) or two self tapping screws to hold the heat sink in place.
Are there two mounting holes on board? If so, 531102B02500G tends to be a popular heat sink due to two press fit posts that are soldered to board to hold it in place.

Very popular extruded transistor heat sinks are 531002 at 1" height (13.4C/W), 531102 at 1.5" height (10.4C/W), 531202 at 2" height (7.5C/W) and 531302 at 2.5" height (8C/W). Bigger the better.. as long as it physically fits.

For suffix --
B00000G means no support provided.
B02100G means support posts are installed along with a 0.05" standoff to slightly raise the heat sink off board.
B02500G means support posts are installed and is mounted flat to PC board (no standoff).
Lots of people use the B02500G and if they need standoffs, they put a mica insulator washers on the posts.

My favorite is the 529802B02500G at 3.7C/W. Really big chunk of aluminum that needs a lot of room.