AmigaOS - Zirias/cc65 GitHub Wiki
If you want to use the current cc65 code base you have to build your own binaries from the sources - see Getting Started.
If you're fine with some older cc65 code base you can find various ports on Aminet. The MorphOS port is maintained by Stefan Haubenthal ([email protected]).
Execute installer script:
cd cc65-2.13.2/packages/amiga installer Install_cc65