Or better still, what about just machining the rear now UN-threaded diameter down, sleeving/bushing it and then pressing that into the Mk2 body. Cross bore the existing tapered hole and press in a suitably sized roll pin that will secure the gas cylinder to the bush AND the gun body.

Or just thread the Mk2 gun body to suit as per the standardised L4A3 and 5 guns.

As for the total-waste-of-time vent holes under the tapered part where the bipod sleeve fits and rotates. Just machine down parallel and press fit on a new suitable tapered section. After all, it ain't goin' nowhere. I can only imagine and cringe at the cost of new manufacture - with the radial milling for the bipod sleeve, angled rear gas vents etc etc.........