Very sorry to hear, this is a real bummer. This is something that we have seen happen from time to time, somewhat easy mistake to make, as the caps are directional and bored offset requiring correct reinstallation position, but they are not marked clearly from the factory. So accidental reinstallation the wrong way and resulting damage is somewhat common.
Here is the bad news sadly. The camshaft bearings are machined into the head with the caps installed and thus the cap is part of the head's machined manufacturing process. Therefore you cannot just replace the cap with another one off a different head. Doing that will result in a mismatch of the top and bottom halves of the cam bearing as the replacement cap will not be compatible with the head it's bolted to. That will result in loss of oil pressure, damage to cam and head, etc. This would be much like trying to replace a connecting rod cap with one from a different rod, or a crankshaft main bearing cap, etc. They are integral to the component in question, part of the manufacturing process, and not able to be thought of as interchangeable off-the-shelf parts, like a water pump or a fuel injector would be, unfortunately.
That said, there are still options. You CAN replace the cam cap with one off another head IF you have the head line-bored by a machinist afterwards to ensure correct fit and running clearance with the camshaft. Common enough procedure that most any machine shop should be able to handle. Or, you can also replace the head with a good used or new one. Maybe your headgasket could use a refresh after all these years anyway, or the head use a valve job.
Brand new AMC heads are very high quality, fully loaded with new cam/valves/guides/etc, and quite affordable. As I understand it they are fairly easily available in Europe -- look on ebay, etc. This could be a great option to fix the issue while also giving your engine a nice update that renews a lot of wear items.
Unfortunately all of the viable repair options mean removing the head from the engine. That is not such a big deal though -- main effort is working with the timing belts upon reassembly, and I am guessing if you have cam bearing caps off that probably means you had the camshaft out, thus you're doing all of that already as it is. Yes?
Hope we can help you keep it on the road, don't throw in the towel, it's a pain and unlucky for sure, but a solvable problem that gives you some opportunities to do other freshening.