The Triumph had the original chain when I traded it but like you said it's dirty and requires maintenance that a belt doesn't especially if you ride in the rain. The Thunderbird gets a belt so I thought by now Triumph would have switched the Bonnevilles to belts.
2011 Triumph America (10/2011 to 07/2014)
2012 Harley Davidson 1200C Sportster
2014 Harley Davidson Dyna Wide Glide