Since then even more CPUs are working on ICE40:
https://github.com/cliffordwolf/picorv32
Or a really small and slow one (smaller than J1, but needs more RAM): https://github.com/combinatorylogic/soc/tree/master/backends...
It uses 1521 LUTs with Yosys. The icestick is a HX1K which has 1280 LUTs I think.
It runs on the open iCE40 toolchain, so nope.
Since then even more CPUs are working on ICE40:
https://github.com/cliffordwolf/picorv32
Or a really small and slow one (smaller than J1, but needs more RAM): https://github.com/combinatorylogic/soc/tree/master/backends...