The mini assembler was built into the ROM of all enhanced //e's
If all you want is LISP, burn it into an EPROM and replace the stock one.
I don't have access to my Apples at the moment, so can't check the details, but I'm now curious to know exactly what might have changed between various II+ - what did others gain that we were missing, or why did we have room for it when they didn't...