Checking those LM393 in circuit should be quite easy, even with simple DMM, testing SN75158 is equally simple.
If these are fine, I don't see a reason why to swap, a good cleaning the PCB might be more important
https://www.ti.com/lit/ds/symlink/lm339.pdf?ts=1592432366109&ref_url=https%253A%252F%252Fwww.google.com%252Fhttps://www.ti.com/lit/ds/symlink/sn75158.pdf?ts=1592432187790&ref_url=https%253A%252F%252Fwww.google.com%252FAdjustment screws are above the lens on the Heidenhain ones.
It goes like: Lamp, beam adjustment screws, lens (collimated beam), stationary grid glass plate, moving glass scale, photodiode assembly.
That stationary grid glass plate is to increase the interference, so to get more light than with just a single slit.
The screws are there to decrease the light, so that the light that every photocell gets, can be adjusted, to optimize quadrature output.
Sure you can probably swap the photocell/electronics sections between scales, but it would probably still require alignment/adjustment and you risk breaking stuff doing so.
Have you checked the light source, is it LED, lamp or laser diode. IR or visible? IR should be visible on camera.