Take a look at db killers for motorcycles, they will be available on eBay, not sure if the dimensions will be ok for your exhaust, but if not it should be easy enough to have something fabricated, basically a baffle tube that pushes inside the existing silencer.
Like these
https://www.ebay.co.uk/itm/Universal-Motorcycle-Exhaust-Can-DB-Killer-Silencer-Muffler-Baffle-42-45-48-60mm-/111868901612?vti=&ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F710-53481-19255-0%252F1%253Futm_source%253Dtwenga%2526utm_param%253DeyJlcyI6MCwicyI6OTcyMDIzNywiY2kiOiIwMzdhMGU5MDMxZjQzNzZiOWJiMWYzMDM3MjgxYTkwZiIsImkiOiIxMTAyOTEzODQ2MzMyNDAxMjg4IiwidHMiOjE1MzcxOTUzNTMsInYiOjMsInNvIjoxNTAwLCJjIjoxNzgwMDB9%2526utm_campaign%253Dtwenga%2526utm_medium%253Dcpc%2526ff3%253D4%2526pub%253D5574631662%2526toolid%253D10001%2526campid%253D5338243305%2526customid%253D037a0e9031f4376b9bb1f3037281a90f%2526mpre%253Dhttp%25253A%25252F%25252Fwww.ebay.co.uk%25252Fitm%25252FUniversal-Motorcycle-Exhaust-Can-DB-Killer-Silencer-Muffler-Baffle-42-45-48-60mm-%25252F111868901612%25253Fvti%25253D%2526srcrot%253D710-53481-19255-0%2526rvr_id%253D1671141675746%2526rvr_ts%253De7fbb46d1650ab1de8c331c3fff29576&ul_noapp=true
