Something like this would do: http://www.ebay.co.uk/itm/330862268131 (i won a pair of 1.8ghz modules for £10)

you need to remove the old cpu's and drill out the mounting holes with a 6.5 / 7 mm bit (double cheack that, i may be wrong there), you may need to countersink the holes so the mounts go in easier when your refitting the modules

thern it's pretty simple, just clean and add new paste to your old cpu cards and screw them on

The only modification needed is drilling out the holes, apart fromt hat they pretty much fit right in