Nice to see someone making their own crafting system!
Anyway, you need a free build icon ID, and just to be certain also a new texture ID (you can changenthe texture later, but it's best to have a custom one with the custom ID tag). The reason for this is that the build icon can't already be taken, and 513 definitely already is taken.
It actually is a lot easier to this this while also using an editor like notepad++ to get the block ID right. The first custom ID is te same as the texture ID listed in the example custom config, and it is the build icon ID too. I think the first ID number (so for custom_000) was 1972. For custom_001 the number ID then is 1973, for _002 it then is 1974, etc.
Why? The IDs are kind of hard to select in the block editor.
I hope this helped.
Ps: custom factories aren't possible without some coding at the moment.