This brought to my attention by @FREEZE64UK earlier today - a version of Sinclair BASIC on the C64 by Whitby Computers circa 1985. It's more than a port; this is a translation of the ZX Spectrum ROM to 6510, with "near-perfect BASIC compatibility"...
https://www.c64-wiki.com/wiki/Spectrum_Simulator

So let's fire it up and find out just how close it gets to Spectrum BASIC...

Okay, so far I'm impressed. And I must admit, slightly weirded out, seeing this on a C64. Single-key entry of keywords, as per Spectrum 16K/48K BASIC, and that Sinclair font.

Well, that seems to work okay. I think that's also a 32x24 screen too.

I wonder how well it works with graphics...

Trying to find the corresponding key on the C64 is fun! Luckily I had a Spectrum nearby so that I could work out which key to press...

Ah, here we go. It's a bit slower than the Spectrum, which is already a fairly slow dialect of BASIC, but there we go. Circles.

I wonder if it has attribute clash? Only one way to find out...

Ha! Brilliant!! It does. For those of you tuning in late, this is Sinclair BASIC running on a Commodore C64. This was written back in 1985. See the thread above for more details.

There are clear limitations to this. It's not a Spectrum emulator, more a port of Sinclair BASIC to the C64. So machine code will not work, as it will be Z80. I wonder how different the memory map is...

Ha! I was not expecting that. Blooming brilliant!! So the author has emulated the Spectrum screen ram from 16384.