WarCraft II runs on 68k, if you have a very fast 68k
Pararena (the 2.0 version) has head-to-head network support and should run fine on a portable.
Spectre and bolo were already mentioned, but I have to plug them, both are top notch. As young kids we used to play bolo as if it were minecraft, building out elaborate maps and towns. A lot of depth and ways to play that one. While Spectre is a great for "pick up and play a round".
If you get into PPC land, Future Cop: LAPD has an astonishingly fun networked head-to-head mode. There's tons of options for PPC but that one is a lesser-known favorite that my brothers and I spent countless hours playing back in the day.