I bought my pair of takedown pins from user
pcwgr1 via the EE. Also sells stuff on that other site where stuff gets bought and sold but we're not allowed to name it here. They sold me a nicely made 3-mag pouch as well. Very happy with the pins, but I don't know if they're genuine HK or not, and don't really care as they work well in my GSG-16.
I also modified the bolt carrier stop plate such that it slips into position with a little knob on the back to handle it, instead of using another cheap screw as it was stock. It is retained by a block of plastic I made to keep it there, which gives just enough room for the bolt carrier and for the end cap at the back. I can get the bolt out in under half a minute for cleaning without any tools, which makes this gun much more satisfying to use. Really dislike all the factory screws. But the takedown pins help a lot, leaving just that one screw for the stop plate needing a tool if you don't want to get into making a complicated part with holes for the guide rods.