In the early 2000's, Teemu Kalvas created "Lisp 500": 500 lines of C (plus a .lisp file that provides a library and goes as far as featuring a compiler, which works by emitting C).

I can't find the sources for this.

Incomplete, no .lisp file: http://www.s2.org/~chery/projects/lisp500/

Doesn't connect: http://modeemi.cs.tut.fi/~chery/lisp500/

The most accessible source for this currently seems to be a "lisp5000" project in GitHub: https://github.com/jackpal/lisp5000

This is a derivative work whose C code has been blown up to 1600+ lines.

Look up femtolisp. It's a fast interpreter that was around 1kloc at one point. Julia is built on it. It was LISP internally last I checked.