We offer this AM29F040B FLASH ROM that can be used on a Harlequin 128 rev 2D board.
It contains the ZX Diag ROM tools, and included is a leaflet with information about how to use it.
The chip can be erased / overwritten many times (see the information beneath).
You can program (flash) this chip on e.g. a MiniPro programmer.
Get more info about where to obtain one here.
How to organise the ROM contents to program onto the chip
More info how to organise ROM images (up to 8) in this chip, is explained here:
Disclaimer: we are not allowed to ship chips with any Sinclair ROM on it, since that IP is owned by Comcast.