MDW Guns is importing the Impulse Gun Barrels again
There's not much to the slide, LWD sells the 20LS you just have to put the guts in it.
You need a backplate, striker - striker spring, sleeve and spring cups, recoil spring assembly, firing pin safety & spring, extractor, depressor plunger, bearing and spring, and a barrel and sights.
You can get a 10" barrel for the G20 from MDW Guns:
http://mdwguns.com/IGB.html
it sounds like a lot but it's not when you detail strip it most of that stuff stays together . You have your RSA, extractor, barrel, striker assembly, plunger, firing pin safety and backplate.
I was going to do all this stuff to a G34 but then Glock made a batch of 17L pistols so I just bought the 17L.
I'm still going to put a longer barrel in though...