Back in "the day", there was a replacement ROM for the ZX81/TS1000 that turned it into a FORTH machine. Was a cheap way to learn FORTH when the host machines were deep discounted near the end of their run.
The manual was great for its time, a little dated now but still a decent read [2]. It was reprinted in 2018 and still available [3].
[1] https://en.wikipedia.org/wiki/Jupiter_Ace
[2] https://archive.org/details/vickers_steven_jupiter_ace_4000_...
[3] https://www.amazon.com/Jupiter-ACE-Manual-Anniversary-Progra...
The register sets are roughly comparable.
The hard part is dealing with interrupts (which FORTH does not have any particular method of handling), if you want to do high level (so in FORTH) interrupt service routines you're going to deal with the CPU state in a tiny bit of assembly and then run the threaded interpreter loop until you're ready to exit the ISR. IIRC this was much harder to do cleanly on the Z80 than it was on the 6502 (or maybe I just knew the 6502 better).
FORTH's tiny language core means you are abstracting out the CPU quirks very early on to the point that it is almost irrelevant which CPU it runs on. The lack of a clear dividing line between 'system' and 'user/application' programming makes it ideal for control tasks but it is hard to remain disciplined as the line count goes up.