Seconded, along with Commodore Vic20 – which, along with Commodore +4 (which is the 64K version of the Commodore C16) is part of the VICE suite of emulators, which I’ve compiled with no problems on the Pi. Should work with X64’s settings only slightly tweaked.
I’m no expert on BBC Micro (Acorn Electron was the cut-down version of the BBC Micro) emulation but http://b-em.bbcmicro.com/ seems to be the best bet for that.
plus 4 / vic20 is supported – just they don’t have their own platform folders currently. with the 3.0 beta you can switch to vic20/plus4 instead of x64 (as you mentioned, they are included as part of vice).