I think I built the first hp300 tree on a HP 735 using the GNU binutils
and whatever the current version of GCC was at the time.  It's not that
hard to set up.  Mainly:

1) Copy the include files into a separate hierarchy on the host
machine.

2) Copy the .mk files, and port our make(1) to whatever weird OS the
host machine is running, modifying the system include directory as
appropriate.

3) Add the appropriate GCC flags.  (You can get this effect just by
setting DESTDIR in your environment now; at the time it was a bit more
complicated.)

Obviously you have to build a set of cross-compilation tools first.

Alternatively, you could use an existing binary set from one of the
current m68k-based architectures.