My silencer looks fine but sounds like a rattle when shaken as there must be that much rust in it !!!!!
What are my choices to fit the standard exup downpipes. (Road not race)

Can you take it off and shake the debris out the end? You can see how much rust comes out, etc. You could strip your silencer and have a look inside, maybe the baffle has corroded?

A guy on another forum had a major rattle from his exhaust, when he took it off and turned it upside down several of his tools fell out (allen keys, spanners, screw drivers, etc). Turns out his young son had been posting his tools down the silencer.

Any can with the correct link pipe.
They are not specific just because of the Exup.
Pick something that is the correct scale for a 400.
Plenty of options for the 600 that can be (see what I did there) substituted.

If you keep the can sensible you probably won't need a lot doing to the fuelling but if you go for a silly bean can thing you'll need it set up, which can get expensive.

If you do end up needing a can, I've got a spare viper road can in the garage that's in good nick. It would need a link pipe making up though as I don't have one spare.

Hi, I have a standard R6 silencer on my 3en1, I had to cut down the output pipe of the exup valve and make a bracket to get it to fit right. But its sounds lovely now.

Cheers
Ian