> Can a person just replace just the white plastic where the screw head is?

Chris:

Get a complete set--if one is brittle, they all are.  It
isn't worth messing with more than once every ~20 years....

Replace one at a time.  Adjust the new one same as the old,
remove the old, snap in the new.  Your headlight adjustment
will hardly change.

I did mine last summer.  Bought them at the dealer--special
order, 3 days, sales tax, no shipping.  Prices very similar
from our 3+ vendors, plus shipping, no sales tax, more than
3 days.

If you have ETKA, study 138-05 to make your list of part
numbers and quantities.

The hardest part is sorting all the adjusters to match each
light.  Cracking out the old and snapping in the new is
easy.

Richard
Boulder