I would get rid of the Landmark Paint saver if mine came out and the paint wasn't messed up. You will have more options for caps not using the paint saver ring. Otherwise JBweld on the threads and screw it back in. Landmark makes a key that will fit there paint saver, pretty cheap. Check J&P cycles.
I have to agree with RRRUFF, I am not a big fan of the paint saver. I had several problems with mine. First it leaked from time to time, some times after filling up, there would be a small "dribble" from around the cap. Also during the summer it would swell from the heat and be almost impossible to get off (that maybe why your insert came off with the cap). I believe the insert coming out as a blessing.
I'll tell you what I did.
First I removed the insert (lucky yours came out easily).
Second, because I had some very small paint blemishes around the edge of the bung, I purchased a paint saver ring. This ring is rubber and chrome and goes around the very outside edge of the bung. You can find them at most major retailer like J&P Cycles (I think I bought mine from ebay). This ring covers any blemishes, prevents getting gas on the tank during fill up, and doesn't interfere with any after market gas cap.
Third, I purchased an aftermarket "custom looking" gas cap. The cap looks better, I don't get gas on my tank, and my cap is much easier to get off.
In my opinion removing the insert was one of the best decisions concerning my BDM I have made so far. You will find people who disagree with me, and in the end you have to make your own decision but I don't regret it one bit.
I have some pictures but for some reason they won't upload to the site. If you would like to see the pics let me know.
