Possibly what you want is an 'exhaust reducer', which can fit over you sleeve and then connect onto the silencer. You can measure the sleeve to find the diameter (a vernier caliper is best) and the same on your silencer. Then give a few local exhaust places a call to see if they can help. On ebay there are loads of suppliers doing common sizes and you will even find straight couplings where diameter is constant (stainless is not too dear either) e.g:Originally Posted by ANTI_HERO_HULL
http://cgi.ebay.co.uk/Exhaust-Pipe-R...#ht_4302wt_997
Double check the sizes before you order!