So if we ever 3d printed a shell of a certain cobra head or yard light, or even better just gutted one for the test. We take the shell, make a mold of out it, and could somehow pour aluminum into said mold, do you think we could make a replacement shells for any light we mold?

This is probably possible. Sand casting materials are relatively accessible. The amount of effort involved in this would rarely be justified, but with a few attempts you should probably be able to get a usable result.

In the western world, this would be prohibitively expensive. You can try to DIY, but furnace equipment isn't cheap, and you'll be doing a lot of trial and error.

Your best bet would be to ship an example to Pakistan and ask if they can cast 25 pieces based on the example you sent there. There is a shitton of tiny foundries that sand cast stuff by hand.

If you offer them 50 euro a pop, they'll likely jump on the opportunity to make a year's wages in one sale. Perhaps you can get it cheaper even, because they won't have to make a mold if you ship them the original.

Quality is not the best of course, sand casting is a rough process. IF you want it perfect, you're better off 3d scanning and CNC machining it.

Interesting… I was curious in case someone could ever make custom or rebuilds for trashed streetlights. Thank for your response!

It's definitely possible. With today's tools and machinery, you could literally 3D scan one, and have a CNC machine carve one straight out of a block of aluminum

I'm interested in the subject of reproducing old lamps myself too!

Lamps or Streetlights? 

As mentioned from other comments, sand casting would be the cheapest way to do it but the casted parts aren't as high quality as what you would get from a die casted part. Now if you design for looser tolerances and then post process the casted parts by machining critical features afterwards, you could probably get something workable.

Unfortunately the NRE and tooling costs for a die cast would be quite expensive, easy to amortize over millions of units produced, not so much for a handful of units.

Reflectors are a bit harder, they were typically hydroformed with special tooling.

The glass would probably be the most expensive to remake, most luminaire manufacturers did not produce their own glass, instead subcontracting the design and fabrication to Holophane or Corning or by selecting a off the shelf design from Holophane.

I believe custom casting services are readily available in the world of artistic sculpture making. Otherwise, for many luminaires a replica case can be 3D printed from some relatively heat-resistant plastic like polyamide.