The 'Enfield Musket' (good pun there!) was built by Aniket Vardhan, an Indian living in the US. He 'officially' went to the US to study for a Masters in Industrial Design, but claims the real reason was that he loved the sound of Harley Davidsons!
Looking at the finished bike just illustrates what a good designer and engineer Aniket is - I hope he got his Masters!
website has lots of details of the bike's construction, along with pictures and videos. In this video, he explains the silencers he's fitted, then starts the bike so you can hear the superb sound it makes. Great stuff!!