The reason that there is no Contiki for 68K is that it is very hard to deliver.

A group of developers who did have a lot of time and inclination actually delivered TCP/IP on a Mac 512K. And it probably worked much better in the day than when I tried it: http://www.vintagemacworld.com/512Knet.html

Those developers know that we would love to have source, but finding it is a challenge.