Here are expensive, but very nice solutions:
https://macmachineguns.com/product/grey-ghost-upper-receiver-m119/
https://www.max-11.com/509.php
https://www.max-11.com/900E.php
Other idea:
Find a gunsmith to tap your upper to support the...
FYI Just pulled out a NIB SWD M11A1. About 75% of the buffer was in the box as tiny black pellets, 10% still inside the gun, 15% where it should be, stuck on the recoil spring.
https://www.rockislandauction.com/catalog/1048/class-iii/
Click on the item to see the realized price.
Not many surprises, but this one stood out at 94K!
https://www.rockislandauction.com/detail/1048/2678/beretta-model-12s-class-iiinfa-fully-transferable-machine-gun
Might try cleaning the mag, then some dry lube. Did you switch from brass to steel case, bit more friction there. You could add a filler block to create more spring compression (and loose some loading capacity) to test the spring fatigue theory.
I had a couple of the BWE swing swivels, mine had e-clips. I will say the AKarms swing swivel are much closer to the original than my BWE ones. Good job.