America's & Speedmasters are a bit tricky to convert to aftermarket/custom tail lights because of the separate lamp assembly and license plate holder (assuming you do not want to bob the fender). For example if you used that Harley Beehive tail light, you would have two license plate holders. You could unscrew the bracket from the beehive, but you're left with an unsightly mounting tab plus a little of the indentation visible on either side.
I would suggest logging onto
www.bonnevilleamerica.com and ask if someone would sell you a stock take-off lamp. I'm sure you will have many takers.
/Mike