If you install complete compliant headlamp assemblies other than the original kind (modular self-contained compliant optics such as the Hella units, or headlamp assemblies from a different kind of vehicle, etc.), and the vehicle winds up with all required lighting and reflective functions, and they're installed such that they can be aimed correctly, you're legal. If you open up your headlamps and install optics and components (drill, cut, glue, screw, etc.) that weren't originally there, then (attempt to) reseal the headlamp, you're not. Even if the optics you install produce compliant beam patterns, by opening up your headlamp you've undone all the work to make the assembly vibrationproof, resistant to water and dirt entry, etc. And internal bezels and lenses often exert influence over the total light distribution via internal reflections, so if anything inside the headlamp changes, so does that, and you can easily get a noncompliance. Of course if you send your homemade headlamps for compliance testing and they pass, then you're legal.
