Ive taken possession of this mystery Buggy and am beginning my inspection and restoration work list. Found floor pans, seat rails, seats, and various interior parts needing replacement I have taken pictures of the rust and cancer. The only cool finds were a 1981 Dime, and an alternative windshield (user worn) system. This buggy has gone to two nephews in a row and I will be honored to ensure the tradition is perpetuated. This is the beginning of my record web journal to document my stewardship efforts.

Things I've noted.
Chassis is a 1965 standard beetle.
Engine appears to be a 1961 1200.
Fiberglass body is of unknown origin
Windscreen width is 42"

I am currently comparing floor pan replacement options. Is a square tube framed and sheet metal fabricated floor a better option for safer more secure seat attachment?

Removed front "hood" panel and inspected fuel tank and whats left of the instruments. Still no clue to Manufacturer.

After more research I've confirmed the engine is correct for the chassis

Recent activity. Have located and obtained donor bug (73 Beetle) with 66 1300 engine that is built to 1600. The power plant will give this a marked improvement over the 1200. I've also purchased sheet metal repair panels for the floor pan and chassis refurb. This will take me into fabrication territory I've not yet tread. so "Geronimo!"

Sounds like you are on your way!  Look forward to more updates.