Hi Scott,
here are my answers step by step:
The engine is not metal but original plastic from Pocher. But it's painted with a metal-colour and airbrush.
"kaputt" has been my first Testarossa that I built in 1995. It has been treated very badly by children, friends and so on. Therefore I decided to restore it and bought an incomplete second Testarossa-kit as a donator of spare parts. Then I found some transkits from Tremonia (Germany), AMG (Germany) and PCM (Italy), all of them discontinued. But I got them - guess the last ones. In addition I took the interior-transkit and the engine transkit from Scaledetails in Las Vegas. But the quality of those parts is a little bit poor.
If you are looking for a good transkit, I can recommend the full transkit set from AMG that is still sold by Scaledetails. That is better than his one ones. And soon there will be on the market a very, very thrilling new transkit from Tommaso Iuele from Italy: https://tommasoiuele.wordpress.com/2020/05/05/ferrari-testarossa-transkit-1-8-may-2020/ But for that I think you will have to pay about 700,- Dollar.
Have much fun
Mathias