The boat in the photo is not mine. But I have worked on very similar canoes.
Yes, mounting new seats can pose some challenges on a boat with those thick aluminum gunwales. I would just go ahead and buy a pair of new wooden frame seats with either web or cane seating surfaces.
To mount them, I would probably use a set of aluminum hangers like these:
The short limb of those L-shaped hangers with the hole drilled mounts the seat. The seat frame rests on top of the L bracket and the hardware shown mounts the wooden seat frame to the hangers.
The long limb of the hanger is attached to the hull and gunwale skirts just below the main part of the gunwale. In order to mount the seats level and at the height you want, you will likely need to trim off a bit of the hanger with a hack saw. If the aluminum of the seat hanger is thin enough, you may be able to situate it between the hull and the gunwale skirt on the inwale side. To do this, you may need to drill out a couple or few gunwale rivets to get the skirt away from the hull. Those rivets would then need to be replaced using a pop rivet gun.
If you don’t want to do that, or can’t, you can mount the hangers inboard of the gunwale skirts. Drill a hole right through each hanger, the gunwale skirts, and the hull material. Use either stainless steel machine screws, washers, and nuts, or pop rivets to attach the hangers to the hull.
Ed’s Canoe, where the photo of the aluminum seat hangers came from, is also an excellent source for new seats.