Got a good '07 muffler to fit to my '89 bike. I notice that the '07 muffler I've now got is about 10mm longer and at least 10mm bigger dia than the '89.
Bigger volume. Looks as though it will fit okay, - obviously as the '89 is a one-peice system I'll need to cut it off about a couple of inches behind the 2 into1 'collector box'. The '07 muffler pipe will slide over this outlet nicely. Just hope the bends are right. Just a little concerned that in terms of mountings, there's a big (heavy) span between the cyl head/pipe bolts up front and the muffler mounting bolt down back... with just that little circlip clamp joint in the middle.
(She'll be right!) ?

(Feel like I'm talking to myself here!) But it's really to help others who may want to put a later muffler on an older bike, and didn't find reference to this in a search, so I'm giving the answers as well as asking the questions.
With the aid of a big glass of very cheapo plonk and my hacksaw, I've just fitted it -(' 07 muffler onto '89 headers). I do find things go together so much better with the aid of lubrication don't you? 

Depending on the condition of the mating quality/roundness of the tubes (header & muffler) you need to cut off your old muffler pipe 35mm to 40mm to the rear of the weld which joins the 2 into 1 'box' to the rear muffler pipe. Fits fine, seems to clear everything.

The new muffler looks a bit 'fatter', and hangs out at a slightly steeper angle - (outwards to the rear), but no biggie, and this can be adjusted slightly with packers/spacers on the muffler mount. The limiting factor being that you can't move the muffler in much 'cos it'll foul the swing arm. Looks fine though.
Re my doubt about mounting strength etc, I'm sure it'll be fine, -once everything it tightened up it's nice and rigid.

Hoping now to have a quiet enough zorst that I can now hear terrible engine noises!
(As you can tell, I'm a 'glass half empty kind of a guy!)

Jools

Ah, runs really nice and sounds much better! And the engine noises even sound less even with a quieter muffler... figure that one out.
I've put a spacer about 20mm long between inside of the frame mount and the muffler bracket which moves the muffler in and leaves about 3-4mm clearance with the swing arm. - Tucked in more and looks like a 'bought one'.
And the carburetion is improved from tickover up the rev range. (It was rough with flat spots before with the old rooted muffler).
(I'll celebrate this with another plonk)
-Jools

(Jools yer talkin' ta yerself again!!!)

Jools just to keep you from answering yourself,  thought I'd say Uh..Huu and just agree with your inner thoughts... LOL 

Cheers Donnie... errr Donnie... err what's that funny coat you're holding... err Donnie...DONNIE... AAAARGH!!!