Your Twenty is timewarp mint! Sweet! So sad to hear some idiot decided to defile her...
Ok - there's one of these on Ebay.co.uk at the moment - The Auction finishes in about a day and a half. If the seller doesn't want to send to the US I'll happily act as an intermediary staging address and send it on for you;
Listing here
Plan B: In theory I actually have one of these in my shed somewhere from my own Twenty... but this may not be true (I was pretty ruthless with the old parts) so I don't want to get your hopes up too much; I'll have a wee look tomorrow - if I recall it's not exactly mint - a bit discoloured maybe and a little scratched. But if I do have it you're very welcome to it. Watch this space for more info.
Plan C - Drop
Old Bike Trader a line and see if they have any - or know of any sources for them.
Huw