I answer myself:
At address 0xf87f (FNKSTR), at the system variables memory, there are 160 bytes where you can change the values of the 10 function keys (16 bytes each).
The strings are in ASCIIZ (zero terminated string).
; Sample code (compiled with sjasmplus)
dw start, newtext_end, start
; Change ...
Generally speaking, Z80 is two to three times slower than 6502 since its fastest instruction is 4 cycles and 6502's is just 2. Then, if you take into account additional instruction bytes and memory addressing, Z80 looks like it's completely behind.
But things in real life differs a bit. Z80 has more registers (7 primary, 7 secondary and 4 additional that ...