CdS cells have a spectral response similar but not equal to human eyes and films, so they do not require almost any filtering but UV. They are used as a current limiting resistor in series with a moving coil meter (another resistors group is used to adjust ISO setting). Then, light intensity rises, cell´s resistance decreases and current rises to a value measureable by the meter. Silicon diodes (any type) require some more complicated electronics. Usually they are driven with constant current so they are biased a little beyond the conduction voltage where the response (light vs. current flowing) is more linear. The voltage developed is either amplified, scaled and later displayed in analog or digital format. Filtering is mandatory as they (as Brian said) respond from UV to IR, and linearizing their response curve is a bit tricky.
All this electronics require a power supply of at least 3 V (usually 6 V) which, together with the full redesign of the metering circuit, turns the modification of the camera into something very complicated not allways worth the time and expense, and sometimes impossible.
I think that replacing the original cells with some new will be an easier, faster and much cheaper solution.
