Make yourself a tapered wedge, hard wood will do it, and whack it in the cylinder end of the pipe. this will expand it enough to make a seal.
Some car garages have a neat but expensive bit of kit that does the same thing, you can find them on ebay, sometimes worked by air, sometimes by expanding bolt.
My way is of course uber naff, but in the "old days" no one cared that much and the thing wasn't worn out enough to need the remedy. Or one just bought new parts.
A totally different mindset which lingers on only in the minds of regressive old gits like myself
and please.....a 'muffler' 
is something woolly worn round the neck in cold climates...its a silencer, and the bit that direct the farts into it is an exhaust pipe not a "header"
email (option): deadsheds@yahoo.com
Before starting I would check that the port is round...they frequently aren't...If it is, the method described by Ken will usually do the trick...
If it isn't you have more of a problem. If you don't want to remove the barrel to have the port machined you can usually true things up with careful use of an abrasive flap wheel.
Once trued if you have not had to remove too much material the pipe can be 'expanded' to suit.
If not I generally turn up a piece that fits the port well and weld that to the appropriately shortened pipe...You can't really see it once the welds are cleaned up...(unless you have a plated system!)
Muffler v Silencer and Pipe v Header?...Which it is all rather depends which part of the world you come from..I agree with Ken though...for the English it should be pipe and silencer!
..Funny how the language changes..Pivoted Rear Fork became Swinging Arm and that has now been shortened to the more 'American' 'Swing Arm'....I still call it a Swinging Arm....Ian
email (option): ian@wright52.plus.com
Strewth....even an Aussie has no excuse for not using the Queens English, or what passes for it down there.
I find that the expansion required to cure the looseness is so little that a bloody good wallop on a hardwood taper is more than enough. Of course you wont be able to use the wood bit again so just chuck it in the barbie.
It needs to be a long taper though, not a short cone.
email (option): deadsheds@yahoo.com
You don't have to go as far as Australia...Just go to Devon, Newcastle or any number of other UK locations if you want to have your understanding of the language challenged... ....Ian
email (option): ian@wright52.plus.com
Just be sure that the pipe is round, and then seal with silicon bathroom paste; let dry for one day, and that's it...
email (option): viaconsu [at] planet'dot'nl
Good point Hans..I completely forgot that...Amazingly silicone sealant seems able to cope with the heat in this application and is flexible....Ian
email (option): ian@wright52.plus.com
....Ian Sarnies anyone?
email (option): spacemonkeym@gmail.com