How can I recreate this in CoBlocks?
HI @mpage,
the repeat forever - block in CoBlocks has the same functionality as the repeat infinity times block in Blockly. The CoBlocks variant is also less error-prone 
You can find the forever block in the Control -> Loops subcategory. Below you can find the CoBlocks implementation of your example:
The position of item - expression block can be found in the Transform -> Position subcategory.
Hope this helps!