Not sure whether the radar tapes are going to be much use regarding the speed at which the aircraft descended (fell).
Don't know the 135's systems, but unless the transponder and therefore the Mode C altitude encoding, is wired to the battery bus, when both generators went offline (either by double engine failure or deliberate shutdown), the transponder would have stopped.
Also given the rotational speed of the secondary radar (2-3 secs/rotation?) and the known high rate of descent from a relatively low altitude, the margin of error is going to be large for radar even if the transponder was still online.