There is no equivalent to an IVA where I live. It also is likely going to become a major government problem needing to process a sizeable portion of the country's vehicles through an IVA like process which requires an in-depth inspection and, a lot of times, comparison against the design standards for which the car was imported from.

An IVA is only checking the car meets established standards - it won't say there's no design flaws or attribute liability. Hence my previous comment about meeting standards. Presumably there is type approval of some kind where you live. Again it's UK centric, I'm not a car lawyer and I've mostly skimmed the page, but there's nothing glaringly insurmountable in what I can find here:

Type approval guidelines

Type approval fees

From what research I've done in the UK at least the responsibility appears to fall on the manufacturer and I can't really see an argument for how it would fall anywhere else as they're the entity producing the product. By using the design there has to be a presumed level of acceptance that they have liability for the product. (I can't just go and make an exploding kettle and sell it and say "I downloaded these plans off the internet and built it, it's not my fault")

In reality they'd have insurance to cover it, the insurance would most likely require the type approval in place.

The supply chain is there, but no one is building older cars that they just have to assemble with aftermarket parts.

If the demand is there and the products are available and possible within the legal framework then it's happening. Believe it or not I watched a video on YouTube in the last 7 days about Tuksedo Studio in Indonesia who do pretty much that.

If it's not doable within the legal framework of a country it's a moot point.