Custom made markers are still out there - made by the handful mad tinkerers with enough knowledge to design and make their own parts.
There's a couple quality anodising places out there, including Slothball (the only guys in Europe I think) ODD and a couple others. Laser engraving seems to be the crazy thing right now, gold plating didn't catch on.
Customising the insides is easier now than ever with plenty of parts for most markers - ranging from lighter rammers, bolts, boards and triggers - and you can install them yourself in most cases. Back then those things weren't so easy to come by and you needed a specialist to install your ACE and wire the Morlock.
As to milling, there's not much meat to cut away from today's guns.