Plumbing – Outside drain – how to replace this grate

I would call it a slip x spigot brass drain street ell. Can't say I've seen one until now. More common is the double slip brass drain ell. Hard to say from a photo, but looks like a short horizontal straight piece could be inserted if you had to use a double slip ell. Perhaps the trap U could be rotated away a bit to buy a little extra distance for this.

First, let me say I'm mystified as to why your question has been migrated to Home Improvement - your question is about a Landscaping product, so posting it on SE Garden and Landscaping was exactly the right thing to do. Unfortunately, it had been moved by the time I logged back on late tonight, so too late for me to give a no vote.

ACO themselves do supply replacement grids, as they're called, rather than grates (well, in the UK) but they need to know the size and whether your grid was an ACO in the first place. If not, they may not be prepared to supply, or may not have a product of the right size and shape. It may be that the drain and grid itself are no longer of a size that is produced, regardless of who the original supplier was - if you can establish which company made your grids (isn't that a name I can see embossed on one side of the grid?), then that's who to contact to see if replacements are available.