The Sunbird 2.0 turbo was actually installed in a few Fieros - less than a dozen - by the aftermarket. (I forgot about the 1.8.)
I used to have copies of the brochures.
https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20050818-2-060169.htmlI think the Fiero was going away, about the time the Quad 4 was coming into its own.
New drivetrains were expensive to get EPA certified and CAFE rated, even back then. I suspect that as sales were dwindling, they had to concentrate on just a few options. (Rumor has it that the 3.4 DOHC would have happened, based on the '90 prototype, which had a DOHC 3.2.)