The compiler already works for the Sun3 as well as the Sun4. Porting to another Unix 68k-based architecture shouldn't be too much of a big deal; porting to MacOS would be more work. Porting to a new architecture is probably something that would take some time and good knowledge about the compiler.
At the dawn of time we wrote an interpreter for Self in Smalltalk, but since then have always assumed compiler support. Toy programs would probably run fine in an interpreter, but real programs would probably be too slow. As an example, when our first compiler was coming on line, and didn't do much optimization, one of our benchmarks took about 6 hours to run. Nowadays it takes 5 seconds. I'd assume an interpreter would take even more time than the original unoptimizing compiler.
Of course, if you'd like to experiment you could write your own Self interpreter.
-- Craig Chambers