Customizing the assets-manager view

You can do this with a custom modal by overwriting the default image asset manager.

editor.Commands.add("open-assets", {
  run(editor, sender, opts) {
    const assettarget = opts.target;
    // code to open your own modal goes here.
   openModal(assettarget)
  }
});

When you want to set an image use this.

const assettarget = opts.target;
assetTarget.set("src", image.url);

I'd suggest the same solution proposed by @khoa-justuno If you need a new modal it will be easier to create the Asset Manager UI from scratch, reusing the DOM elements created by GrapesJS, in this case, would be just a pain to maintain