You can't use a silicon cell because the output is dramatically different from selenium, and, the spectral sensitivity is even more radically different. Selenium is a pretty good match for film, silicon is not. "Silicon blue" cells used in cameras are tiny cells with blue filters intended to correct the spectral issues. Those cells are not usable in a galvanometer type meter because they put out tiny signals rather than output sufficient current to directly drive a microammeter. The SB cells drive an amplifier which then interfaces with the camera's metering circuitry.
In some cases, the only repair a Master needs is to clean the photocell contacts. (The entire back is one terminal, the other is a ring of tinned lands around the circumference of the sensitized side. A springy contact presses against the lands, and if it becomes oxidized, it will build up resistance, and even if the cell is good, the meter won't work.)