I'll dig through my mountain of parts for that cap, and see what I dig up. As to your gauges questions, I'm afraid I'm not much help there. I may own two of these things now, and have had the first one for about 7 months now, but sadly I've literally never once in my life driven a Capri before. Sad, but true. I couldn't tell you how the gauges are supposed to read. I would be quite worried about that oil pressure situation, though. If that gauge is accurate, and you have near zero oil pressure at operating temperature, that's a big big problem. That'll kill your engine quickly. If you aren't super mechanically inclined, take it to a garage and ask them to manually read your oil pressure, specifically at idle and operating temperature. It's not too hard to do yourself if you wanted to learn. You can probably score a cheap oil pressure testing gauge at Harbor Freight or Amazon for less than $20. Someone else here can tell you what you should be seeing, but it'll definitely be significantly above 0. I'd hazard a guess of around ~30 psi.
Same deal with the temperature gauge to be honest. Most cars run around the middle, if not slightly higher. If it's reading 75% of the way up or more, that's something you'll want to look into. These engines have aluminum heads, and aluminum is prone to warping at sustained high temperatures. It's possible you coolant temperature sensor is faulty, or there could be something mechanically wrong with your cooling system. If the gauge is just reading a hair over half way though, that could well be normal on these cars. Sorry I can't be of more help.
Macungie, eh? I was just down in PA on Saturday grabbing the chassis off of Rocketman. Perhaps he has a spare reservoir cap kicking around. I don't think you're too far from him, it'd be a nice little drive. I'll hunt down one of mine though and see what sort of shape they're in. The universal reservoirs do have multiple spots for inlets/outlets, though. The one I used is plastic, and had 4 plastic tabs coming off, one on each corner. You just trim off the edge, slide the coolant hose over it, and put a clamp on. You'd be able to make it work exactly like the OEM one. One inlet from the radiator, and one outlet to the ground.
https://shop.advanceautoparts.com/p/interdynamics-idq-automotive-coolant-return-system-for-compact-cars-mcr-3/6010229-pThis is the exact one I'm talking about.