I have a 750K7 with the same style pipes as you. My pipes are from the states and are stamped as such. The pipes are a sealed unit so the baffles are not replaceable as on earlier models. To find out if the baffles have gone give each pipe a sharp tap with your hand, if they are knackered you will hear the baffles rattle. In use the pipes should be real quite, again if the baffles are shot you will hear them rattle, no mistake. If they sound fine just squirt some WD40 etc down the pipe and around the end and that should get rid of the rust.
As the pipes come in one piece, here in the UK at least I don't know of anyone making slip on aftermarket end caps. I know you can get aftermarket 4into1 systems. Also I believe Dave Silver here in the UK are advertising OEM pipes.
Hope this is of some use to you.