Very interesting. And it is a beautiful shell, indeed! Sunroof car no less.
Alpina was founded in 1965 by Burkhard Bovensiepen in Buchloe, Germany. In 1971, they were a BMW dealership and BMW tuner. Known widely today as Alpina, the firm is now registered as a car manufacturer. So they do, indeed, still exist, and might have some records regarding your car. In 1971, you would buy a new BMW through them and customize it to your needs. Your Alpina options — suspension, brakes, engine, rims, seats, whatever you wanted and could afford — would be installed on your new BMW in Buchloe, and the car would then be shipped to you. U.S. deliveries were rare.
It is conceivable, but also purely speculative at this point, that some of the parts borrowed from your car were borrowed because they were Alpina parts. Stated differently: “Hot!” ?
Due to U.S. emissions standards, it was difficult to import Alpina-modified engines without extensive testing. Thus, most U.S.-bound cars may have received suspensions, brakes, rims, etc., but not engine modifications.
A 2000tii touring sold on Bring a Trailer a few years ago is a beautifully-documented example of one of these rare Alpina imports.
Check out the documents that accompanied this car. A BMW treated to the Alpina experience was expensive!
https://bringatrailer.com/listing/1972-bmw-2000tii-touring-alpina/
I guess I would start by contacting Alpina, leading off with the information from BMW Group Archives.
This is very exciting!
Regards,
Steve