You can pick up the converter at several on-line auction sites. Just type in "colt barrel thread converter". As far as the buffer tube, I don't know. What version of the Colt do you have?
Welcome to the forum. The 50 rounders are apparently from Black Dog, video HERE. The drum mags are available HERE. I don't know if they ship to your location, so you will have to check with them.
Just picked up a used P22, feels really nice in the hand, natural pointer to the target. Have not been to the range as of yet. A friend of mine at work told me his P22 just "blew up" in his face. I am waiting for pictures, but after talking to him, the slide apparently failed during a firing exercise. He caught some debris in the face, and is very lucky as he was not wearing glasses... He returned it to Gander, and they told him it was out of warranty.
Received $$ from some family members for Christmas, so pulled the trigger on the Fire Friday online, was delivered 24 hours later. It seems to be well made. I have had a Kindle e-book reader for a year and like that also. For the price it is excellent!
After looking at the 416's manual, it appears that Umarex used the same internals as in the Colt M4 .22. There is a detailed tear-down/cleaning procedure for the Colt HERE. Should be very similar for the HK.