The copyright on the Spectrum ROM (and by extension its font) is a gloriously complicated affair :)
In 1999 Amstrad clarified the issue as best they could at the time; the ROM image itself is distributable without issue provided an Amstrad copyright notice is attached and there's no financial gain. Since then, Amstrad has become part of Sky, and they've taken the same stance but have also clarified commercial usage - if you want to include the ROM as part of a product, especially if you use the Sinclair and/or Spectrum trademarks, you need to license it (which involves payments to a charity of their choosing). If it's something that can be downloaded onto flash memory for use, the copyright notice is all that's needed. In short, this is why the ZX Uno can exist as a wholly open source device, while the ZX Spectrum Next project is making payment to Great Ormond Street Hospital.
TL;DR - provided there's a copyright acknowledgement it's OK :)
I think later on I'll be getting an original Espruino hooked up to a monitor to satisfy that itch!