There aren't really any GOOD c64-emulators that run at decent speeds on a 68k-based Amiga, as the emulators require a lot of horsepower.
But for spectrum there's an old port of X128 available, which was an awesome spectrum emulator that emulates the regular 48k and 128k spectrums, and even emulates some of the popular russian clones. It ran at full speed on a 060/66, so I'd guess it runs fairly well even on a 040-equipped Amiga.
