The Bren was a pretty good light machine gun, built in England from a Czech design, and arrived just in time for WW2. There used to be lots of them around, not so much anymore. However, there is a place in Boca Raton, Florida, that specializes in putting demilled, cutup Brens back in action, Project Guns LLC. Once you visit the website, you'll be busy for hours. It seems that a guy with basic machine tools and welding gear can do anything he wants to.
Project Guns build many other types of machine guns also. From MG 34s and 42s, to the venerable Vickers, you too can own a real machine gun from classic times (in semi-auto of course). In this following video from Project Guns, you can watch a demilled receiver come back to life... It takes a lot of work, and a huge amount of skill to pull this off. If you have those skills, you can buy jigs and tools to make the build process easier, or if you prefer, they have guns to sell.
Many more videos on Project Guns site...
Of course, all this begs the question as to how strong these guns are, since they are just pieced back together. It would be possible to do this and sell the gun as an original, but inspecting the inside of the receiver should give it away. The fact they can bring them back to life at all is just amazing.