To build the Z80 simulator for MUSIC on Hercules or Sim390:

Log in to a suitably privileged MUSIC account such as $000.
Create a library to hold the Z80 Simulator files, $Z80: for example.
If not already done, a UCR record must be created. Give the command:

UCR

and at the ? prompt, enter:

SET,CODE='$Z80',UCR=-1,-1
END

Next, restore / download the source files into the $Z80: library (if
some other location is chosen, appropriate modifications must be made to
Z80.FORTG, Z80.LKED and Z80.EXE)

On the MUSIC command line, give the command:

$Z80:Z80.FORTG

When compilation is completed, two files should have been produced:
$Z80:Z80.OBJ and $Z80:$Z80.LIS. Review $Z80:Z80.LIS for errors and/or
warnings. A single warning "IEY038I SIZE WRN." is expected and should be
ignored.

Next give the command:

$Z80.LKED

(Press return when a sneaky More... appears on the bottom right with
no other indication that the link editor is sitting there doing nothing,
just waiting for something to happen.)

This will produce the load module $Z80:Z80.LMOD which is loaded and
executed by $Z80:$Z80.EXE which in turn can be called from $Z80:Z80
The latter is an interactive REXX program which can build appropriate
command parameters for the compiled program and help the user manage
their Z80 source and object files. Unfortunately, the submit to cross
assembler facility is not available so some other method of assembling
Z80 assembly language into hex files suitable for use with the Z80
simulator will be required. Maybe someone else has a Z80 cross assembler
for MUSIC stashed away somewhere?