exhaust expansion chamber on 4 stroke ?
 

I have a 1981 yam SR250 which has recently destroyed its exhaust system. It is in good condition other than the expansion chamber which has now rusted beyond repair. So I'm thinking can I just cut off the expansion chamber and weld a plate over the hole? see photo of same I found. The expansion chamber is the middle bit and sits right in front of the rear wheel so gets all the crap all the time.

Attachment 17732
2 strokes make good use of these chambers I know, but on a 4 stroke? Just wondering if you guys & gals think it will make much difference, or would I need to do "other stuff" ?
Loosing a tiny bit of hp is hardly going to be noticeable (it's not used for track days) but I wouldn't want to cause any damage for example by running without this chamber. Finding a replacement without the same rust issue is almost impossible and if it is possible, more than the bike is worth in £'s.