I've reused them, mostly with success...but not always. Look at it. If it looks good, it probably is. If you see it looking burned from hit gas leaking or brittle from age or mechanically damaged, replace it.
Any turbo shop can sell you one. They aren't specific to Perkins or even that turbo-- it is a very common mount.
You may have another gasket between the adapter and the block. That is Perkins specific, or st least not super broadly used. But, they can be had mail order for a few dollars. If you pull it, which you likely don't need to do, the same inspection applies.