> Is F.O.R.T.H. available for Amiga?

Yes, it is. That's what Aminet has for m68k:

http://aminet.net/package/dev/lang/F68K
http://aminet.net/package/dev/gg/tile-forth-bin
http://aminet.net/package/dev/lang/Jax4th
http://aminet.net/package/dev/lang/AForth
http://aminet.net/package/dev/lang/Forth
http://aminet.net/package/dev/lang/cforth

There's even one implementation for PPC (PowerUP, so should be MorphOS compatible):

http://aminet.net/package/dev/lang/PPCcforth