Replacement or repair headlamp

[RLiotta]

I have a 1985 4000S Quattro

One of the headlamps is orig equipment- the other was replaced by State 
Farm after exploding from a direct rock hit.

The orig equip lamp is in sad shape--  cracks and small holes (from 
pebbles), water (which I have sucked out- and tried to stop leaks with 
silicon).  The light pattern is poor-- very diffuse.

Where can I get replacement lamp unit-- or better yet--  better 
technology??  I would need the ENTIRE unit. 

Or---  is the a glue-- better than silicon--- to repair cracks and bring 
back a good light pattern?

Thanks very much----  Rick