The only thing a Subaru guy would do is possibly change the oil or bleed the cooling system. Maybe change a timing belt after many thousands of miles. They're pretty maintenance-free.
The rest of the car is basically a Beetle though, probably a not a Subaru guy's specialty.
JPS uses aftermarket ECU as well, so not a stock Suby ECU. So does Greg at Vintage Motorcars. JPS is an A coupe, and Vintage is a pre-A bent-window.
Beck/Special Edition is making a 356C coupe with Subaru engine and ECU. The first year or two are all spoken-for and mid-engine. But they are planning to do rear-engine in the future.