Try cleaning up the cpu/gpu dies and HS and add new TP.
If that doesnt sort it try reflowing the southbridge (NOT hairdryer or towel trick), I had one console had 2rrod with perfect heatsinks and spread of TP and southbridge reflow cured it, you can also cut the temp trace but then you get no auto control over the fans and the xbox will slowly cook itself to death unless you apply a fan mod to it.
Fitting another heatsink isnt going to do anything, even if the one that was on there was partially warped it would still work for a while but the temps would just rise and rise till it errored out to 2 RROD. Hell you can just sit the HS's on the chips with a little tp on a bare board with no pressure for testing so its not going to be faulty Heatsinks.
You shouldnt really be poking around in it tbh, if its come as not described then you should head straight to a SNAD claim with ebay if the seller isnt responding.
Team x crap wont do much, it does repair consoles but breaks them at the same time. you just get normal machine bolts a few washers and a few useless pads for the underside that will warp the board. you could have bought the same bolts/washers from your local hardware shop for cheaper.
If you are going to keep it and try to repair then keep the X-clamps on it, ive noticed in my own consoles that I get a far greater success with x-clamps.
Cechc03: Ylod repaired 18/01 Kmeaw 3.55
Cechc03: 3.55 OFW, Ylod repaired 26/01
Cechc03: 3.50 OFW, Needs laser.
Cech2003b 3.15 OFW Not modding till warranty runs out