I think if these were ported to IF6 and compiled into Z8 machine game the parser would be improved nearly for free but they woudn't run fast enough under an 8 bit machine. But for DOS, Amiga, Classic Macintosh and even the ESP32 it would be more than enough to run these.

The parser in Z-machine games comes from the game, not from the runtime, so the Z-machine version is completely irrelevant and the only reason porting to Inform 6 would make a difference is that you'd be linking it with a different parser.