Skyscraper supports https://www.worldofspectrum.org as a scraping source., which also lists ZX81 programs.

Got there eventually. For anyone else having problems:

Go to Settings>Input and set User 2 Binds to Sinclair Keyboard, and User 1 Binds to Cursor Joystick. Then go to Quick Menu and click. Save Core Overrides. That'll set you up for basic registering of inputs without having to repeat it every time you load a game.

For individual games that aren't pre-configured with those settings, go to Quick Menu>Options and set the individual controls, then save the config file for that game from the top of the same menu.

@tredelijkheid said in Trying to add a new system - stuck already with the system.svg...:

someone could look at the file and tell me what is wrong. Again, it looks quite OK but in ES it does not.

I quickly checked the file and it seems ok, as you said.
I noticed the image "size" (dimension of the page in inkscape) is quite big compared to usual system svgs. ES does scale it but sometimes that scaling has issues.
I would suggest you open one of the existing ES system.svgs (or any of my custom system icons) , save it with a different name, delete all objects (making it "empty"), open your sinclair svg, select all objects and paste them in the "empy" svg, group all objects and save.
That should fix your problem.