There is a white lever preventing insertion of cassette. I can't
see anything to make it move. Its either out of position or
something else is broke or missing. If I remove the lever it
will otherwise work except the mechanism may be unusable.

I assume the "white lever" is to the right, a few inches into the opening.
Sony calls it the "prevention lever", though what it "prevents" isn't clear.
(It might be the erasure prevention lock. If the record timer has been set,
inserting a cassette with the erase-prevention tab knocked out will cause
the VCR to immediately eject the cassette.)

I would suggest opening the unit and looking carefully at what happens when
you load a cassette. Other than that, I don't think I can help, as I don't
have the depth of experience of Arfa, et al (whoever he is).

This part does not have a part number. It's a component of a larger
assembly.

The SL-HF900 was a great product. Very much worth getting working.