Could you baffle it from the other end as it were?
These are meant to go on the pipe before the silencer.
https://www.classicbikeshop.co.uk/1-...-baffling.html
There are three sizes available. You would have to chop off the reducing sleeve part of it and then TIG it onto the end of the exhaust pipe assuming you could get one to match the diameter of your pipe.
That's probably more faff than starting fresh with some correctly sized perforated tube?
http://www.everyexhaustpart.com/cart...forated%20tube