How can I run MAME on an iMac G4 (PowerPC)
You can get PPC version 0.103 at MacMAME

You'll need a ROM set of the same vintage, so search for "MAME 0.106 reference set" or similar.

You want either a specific PPC build or a Universal build. All builds labelled 32bit or 64bit are Intel only. sdlmame0140-universal would also work but compared to MacMAME it's more difficult to configure, which also applies to those in your 2nd link.

Finally, I would like to recommend you play SEGA's 1984 classic Flicky.

Have fun!