I don't personally know of any repair shops in your neck of the woods, but re-ring separation and ply separation are generally easy DIY repair jobs if you've got plenty of clamps and an appropriate glue.
I've personally had good luck with Titebond II and Titebond III wood glues, both of which are commonly available. Work the glue into the separation and clamp in place. I use cheap, common c-clamps, placing a little bit of scrap wood between the jaws of the clamp and the drum to prevent scarring the drum. Once you've got the clamps in place and cinched down, wipe away the excess glue that squeezes out with a damp cloth.
If you're worried about pulling the drum out of round with clamps, you can use a turnbuckle or another device to exert outward pressure at various points around the clamped re-ring while the glue is setting up. Install as needed, turning the buckles just enough to nudge the shell's diameter into consistency at all points. Again, use a piece of scrap wood between any metal tool and the wood of the drum.
The glues I use aren't what they would've used at the Slingerland plant. If you want to go the extra mile for authenticity/originality, you'd need to be willing to put in some extra labor to work with hide glues. If you've got a local luthier (string instrument builder/repairman), they'll be experts in working with the stuff and might be willing to work on your drums. It'll be an easy repair compared to what they're used to working on.