There is already an ISA interface for Amstrad PPC done by Retro Theory. Retro Erik let us discover in a video another ISA interface for Amstrad PPC by Enide. You can buy this card in his Tindie shop (assembled or not), the stock is 0 at the moment so join the waitlist to let Enide you want one. More informations on Enide's web site.

As explained in the video, Enide's interface doesn't have a power input like the one by Retro Theory, but the PPC has enough power for a possible hard disk (on a few PPC sold only) and two 3,5 disks drives, so it should be able to provide power to up to 3 ISA boards without a problem. Retro Erik recommends to check the video by Gouldfish on Games where the card has been installed inside the Amstrad PPC. But if one day you replace the internal screen by a new LCD screen, you should check if there is enough power with the ISA interface used.

I was lucky to be able to buy the card by Retro Theory on System.CFG's forum.