Fortunately, it seems I may not need to resort to destruction: checking back through thousands of cat photos on my phone, I realised that I had the sense to take a photo of the key when I first got it, so a replacement is now on order from Kryptonite.
On second thoughts, I'm not confident of getting this any time soon: looking back at my Kryptonite account, I can see that I ordered a spare for my New York lock, having lost one (but, crucially, not all!), in
February 2023. It's been so long that I'd forgotten I'd even ordered it; needless to say, no key has arrived.
If similar wait times are to be expected, I might need that angle grinder after all...
EDIT: Looking through their Facebook page, reports of their customer service are...not good. Think I may have to write off the $20 I've paid for replacement keys and go and spend £25 on an angle grinder from Lidl! The combined cost will be more than I paid for the bike in the first place.