What is the procedure for this, in legal and physical terms?
Replacement fibreglass cowlings for most SEPs cost best part of 10k, and it would seem possible to get replicas made by a classic car body restoration facility. But how would you legalise it?
Under FAA Part 91 you could do it under the owner manufactured parts concession. One article is here. However, what data is needed to support this? Presumably, provably identical materials.
This company is selling Socata wingtips, which AIUI are installed “with a 337” which obviously means a Major Alteration and, absent STC, is likely to be a Field Approval. But then you are buying a finished item which is a different thing in paperwork terms. A PMA is the traditional route for this sort of thing.
What about under EASA?
You’ll know better than most people Peter. What about the “repair” method where a tiny part of the old cowling is incorporated in the new one.
Isn’t this what happens with exhaust systems repaired by welding companies in the US. ie you get a brand new incoloy pipe, you have to send in your old one at some point and in theory there is a small part of somebody’s exhaust in there. Isn’t that how it works?
Trigger’s broom, a.k.a. the ship of Theseus?
What about the “repair” method where a tiny part of the old cowling is incorporated in the new one.
Yes that is one possible way 
Can one do the same under EASA? IOW, can one get an exhaust “repaired” by one of the US companies, sending them just one of the end clamps? Of course the “repaired” exhaust would have to come with an EASA-1 form.
I had to look that one up
This is a very old question of when does repair become fabrication and when does fabrication become manufacture ?
With components the dividing line is clear with metal aircraft the dividing line is more blurred but with composite and wood structure there no dividing line because the repair and manufacturing techniques are almost exactly the same.
As to manufacture of new cowlings the set up cost would be high even if these items came into the category of repaired items.
I would guess that repairing an old cowl to a state indistinguishable from a new item would require a production run of twenty repaired cowl sets to make it cheaper than buying a new item from the aircraft manufacturer.
Making a new cowling is a non-starter for any approved company. Repairing the old one is possible, but will cost several thousand for a specialist, always assuming there is some kind of repair information in the manual. Having an unapproved job done is also possible as long as the company signing out your aircraft is aware. But, it will still cost 50+ manhours for a sizeable repair or 100 to 150 hours to make new (or perhaps more). What is their shop rate?
Interesting. Yes I can see making a mould will be expensive. Setting aside the economics, what is the legal route? Perhaps it is similar to composite airframe repairs, but presumably those don’t involve making a mould.
Now, let’s assume the cowling is mostly intact. Unless the plane is smashed up (then insurance pays) it will be. If you want to make a mould, you will require a largely intact cowling, anyway. So probably in most realistic scenarios the cowling can genuinely be “repaired”.
One common issue is that you get cracks. In the case of the Socata TB, you “can” remove the lower cowling without removing the prop spinner (many maintenance companies manage it, IME ) but it cracks the cowling in the spinner area. How would this be repaired?
Now take a larger crack, say one which runs right across the cowling. How can this be repaired? AIUI, you taper down the material, exposing the layers individually, exposing a good area on each one, and then build up the repair. Socata once showed me how they repair the TB GT composite roof, where somebody opened the door into a strong wing and ripped the hinge out. The material is about 1.5cm thick there. But could (would) you do that on say the ~3mm cowling material?
Peter I could tell you how the repair should be done , but then the whole world would know and We would no longer have a competitive edge.
If you can find a legal way, I know someone that makes ultralight wings from carbon/glass composite materials and could make a TB20 cowling in no time. Much lighter and sturdier than the original fiber glass one.
But he cannot provide any paperwork. I wonder if it would be easier in my case, with an N-registered aircraft?
There is pretty much no legal way to make a new cowl under EASA. If a company had a production approval the production drawings from Tarbes would be required, even if a mould could be manufactured economically. It must be made the same as the original as that is the certified design. It would not be legal to make a cowl from carbon, unless someone had had a mod approve. Mending cracks is straight forward for specialists.
