Hey B & H.
I'm building a Felt Twin right know, I'm using a 4 strokes engine because of the reability in the long run.
I got the bung and cap form a member her, he buildis it especificly for the felts and they fit perfect and have a special design to be JD welded.
About the o ring, if you review the other conversation you will find that the oring will end dry and cracked because of the gasoline.
Check the link for the cap and bung.