If you have half an idea of what you're doing, you could probably just replace the missing screws and run a test roll to see if focusing & aperture are still accurate.
If you have doubts, a good CLA certainly won't harm your lens.

The *BIG* question is, why are you losing screws like that?
It certainly isn't normal. Either someone took the lens apart and didn't put it together well, or it's been subjected to vibration: unpadded contact with an airplane carry on bin is especially bad, but vibrations from other vehicles can also loosen up screws. In that case I'd recommend a CLA as other internal screws and retaining rings could also be loose.