To find any good data, you need to find a library with Aircraft Engines of the World: 1964/1965 still on their shelves. Jane's for that time period might also cover it.
For modelling purposes, I'd reckon that a 1/48 Spey front face would work as a 1/72 Medway front face. I'm plotting to do a re-engined TSR.2 with Medways using the Spey exhaust nozzles from Revell''s 1/48 F-4K kit; the aren't accurate for 1/48 Spey nozzles, being sized to fit the same base as 1/48 J79 nozzles, but I reckon they'll be "close enough" for 1/72 and a low bypass-ratio turbofan would work and be rather more efficient in most flight regimes than a straight turbojet like the Olympus 300.