Hi Shane, sb30inch and CyberJack,
The changeover year for the exhaust shapes on the post-war Atco lightweights appears to be 1951/52. There are three exhausts that were used around this time the first is the torpedo shape, the next is a version of the "pepper pot" that faces forward (rather than to the side) and the last is the one that faces to the side (as shown in Shane's photo). Certainly by the 1952/53 season, the side facing exhaust was the norm. Consequently, it is likely that the exhaust is original (or close enough to it).
As a collector of Atco mowers, I have found that many people who are selling parts overseas are more than happy to sell them to Australians, provided that the prospective purchaser is prepared to pay the postage etc. I have had a number of cases where an item was listed as not being able to be sent to Australia but a quick email to the seller has resulted in a very friendly response and the item being revised so that it can be sold to an overseas buyer. So I would suggest it always pay to ask - the worst thing that can happen is that the seller may say that they are not prepared to post the item overseas. If you have no luck try The Mower Centre, Hailsham, East Sussex as they specialise in old mower parts.
Cheers,
Sir Chook
