TOPIC:

Pretty much what the title says. I know it’s a long shot that anyone would know, but thought I’d ask. The ZRX400 seems to be a reworked version of the Zephyr 400, which shares the same frame as the Zephyr 550, so I think there’s a possibility. 

Related question, has anyone ever been able to import a fuel tank from Japan? It looks like it’s a pretty heavily restricted item. All the auctions on Jauce are blocked. 

There is always a possibility that the zrx400 tank would fit, but you would have to look and relook at pictures of the used tanks to see if the mounting points appear to be the same. But they could “look” the same and still be in totally the wrong place. 
A better bet would to look for the Zephyr-400 which is the exact same bike as the zr550. But finding one that isn’t rusted out will also be an issue. Certain country’s won’t ship used fuel tanks due to residual gas fume issues. I bought one from Canada and shipping cost double because it was labeled as hazardous.
Webike Japan used to sell Doremi kits that for the Zerphyr 400/550 that were entire body kits that made the bike look like the more vintage models. Tank, covers, and tail, but they are expensive. 
I considered it for awhile for my zr550, but I was able to salvage my rusted tank and then found another pristine tank as a backup. 

Thanks for that. Mostly the reason I’m wondering is because I like the profile of the ZRX400 tank, not so much because of any issues with the stock tank I have on my ZR550. I recently purchased a backup 550 tank off eBay, which looks to be in poor shape, but I’m considering cutting it open and modifying it. I don’t know, lots to think about.