Wow - as always, absolutely amazing help and feedback.
BLUSH!
I'm so fortunate to have all your help and guidance on this. GoMog, I was open to all solutions, manufacturing my own or buying off the shelf - my welding skills need work, my joins look like a lump of lava that a volcano threw up so I'll probably play is safe and go with New England Morgan Spares - they've always been a great source of help. With each project I complete I build up more confidence to delve deeper into looking into being more self-reliant, but in this case, I'll probably play it safe (literally).
North American is not the best place to source vintage UK parts. (duh!) Frankly, UK shippers will often get the component to you quicker, cheaper and their knowledge is greater (with exceptions). That being said Larry and Linda (Morgan Spares/ Morgans of New England) have a world class expertise in older Morgans, something that is disappearing every day. They also have the initiative to make/find replacement parts that have disappeared from most Morgan markets. Being American, they will be able to arrange to guide you to an exhaust ceramic coater they trust. The USA, with the majority of the examples of q few legendary Morgan era, is lucky to have them. Automobile expertise is disappearing everywhere. Modern technology, AI and current production cost cutting won't allow it.
That being said, I admit I was hoping you would take a stab at making your own. But buying the manifolds is the safest bet. Depending on price, the rest of the system is straight pipes, silencer/mulffler and optional exhaust tips you can source at any AutoZone, (polished).
I admit to a huge weakness for the Plus 4 SS UK silencers. Effective and pretty.
As far as the air pump goes - it must have been after market, if nothing else, the plumbing joint that connects the pipe to the exhaust says "Made in USA"

- I wish I knew more about the mods on the car, it came with cow catchers and I assume it was used for racing or rounding up steers (or both) .
That is why I added the old Harper's diagram. I have seen some extraordinary bitsa fixes. There was a LONG era where the cheaper you could manage keeping a vintage car on the road the better an owner you were. There are books written on how to do so. Morgans lead that crowd among other British sports cars as it was a very simple but also a very race-winning car. Lots of ingenious stuff, but not Morgan[i] per se. I figure the Morgan community, like any other, is founded on our cars' things in common. Additionally, when you keep learning as you are now, you soon realize that you can do anything to your car and very inexpensively! That doesn't mean you should..
Luckily, I no longer have to worry about smog tests for a car of this vintage - wish they did proper MOTs as this is the only thing they consider as to whether a car is roadworthy over here - the number of cracked windshields, broken lights and bald tyres on the roads is quite startling.
I have a similar situation but with a much newer example. Long story.
Lorne