@Harry's Trains the Heavy Mikado upper boiler was new. The entire lower half, including the tiny backwards motor was reused from the Light Mikado (which was originally a K-Line mold.)
@Landsteiner I think you're right. The Reading T1 was probably the last new steam loco. They re-used a few pieces from the Milwaukee S3, because the boxpok wheel pattern isn't exactly right for a T1, and the rear truck is definitely the same as the S3. So it was mostly new. That was in 2016, four years ago. So maybe we're due !!