I did look, there was one that cost more than new as it included the fork top. But even if they would sell them separately (though I am guessing they do not have the tool to undo the security bolts given they did not) the saving would not be much.
But it seems I may be mistaken anyway. On the bike specific forum someone has said they had to replace their barrel, and they just used either a third-party one or the north American lock assembly. Then, as nivag suggests, they use the original key to work the immobilizer. They still need it for the fuel cap and pillion seat lock anyway.
As a north American lock barrel is only $92.73 from Partzilla (it lists the full lock set as unavailable), even with tax and shipping it should be quite a saving. If they had a full lock set I assume you could buy a blank chipped key, cut that to match the new key, and program it to match the old one. Just a pity the Chinese lock kits on eBay use a different tooth style so you could not do that with theirs.
Cannot say I am optimistic, but maybe the mechanic will be able to fix the lock. But if not then I will see what he has to say about it all, even just getting a cheap Chinese one for now and using the old key as a fob would get the bike back on the road quickly and somewhat cheaply.
Although they do not offer H.I.S.S. in north America, I have no idea why the lock has a different part number there if it is still compatible. But even the bars do, and I cannot imagine any real difference there. Though the price difference seems to apply to everything, so I doubt it is because of any extra technology.
Just picking a part at random which is the same code in both countries, the “HOLDER ASSY., HANDLE (UPPER) NH303M (MAT AXIS GRAY METALLIC)” (53130-MGS-D30ZB) is $21.40 in the U.S. yet £89.20 (before VAT) here. And that is just a lump of metal and four bolts.