My port of the New Typesetting System (NTS), called KeenType, whittles down the Java-based implementation to a few of Knuth's files. Namely, "plain.tex" and "hyphen.tex":
Getting familiar with the fonts required understanding the difference between font metrics (TFM files) and the fonts themselves. To make matters a little less straightforward, Knuth created a special character mapping for indexes into the fonts. It was not easy to find a font that mapped those glyphs exactly. The closest font was BaKoMa:
https://github.com/DaveJarvis/KeenType/tree/main/tex/src/mai...
Getting familiar with the fonts required understanding the difference between font metrics (TFM files) and the fonts themselves. To make matters a little less straightforward, Knuth created a special character mapping for indexes into the fonts. It was not easy to find a font that mapped those glyphs exactly. The closest font was BaKoMa:
https://github.com/DaveJarvis/KeenType/tree/main/tex/src/mai...
This required hard-coding a mapping between Knuth's code points and the actual code points in the target font:
https://github.com/DaveJarvis/KeenType/blob/989dbe26f68eda75...
Plain TeX has a lot of moving parts, so it's great to see more people writing about the topic.