so you bought a bike that there really isn't a lot of aftermarket support for and you are complaining that the "universal" parts you are finding for the bike aren't really a good fit for the giant 500+ lb 1980's superbike you bought. I mean, are you really surprised? most "cafe racer" parts are made for tiny 1960's british bikes, and some of the stuff out there for later japanese bikes like xs650s and cb750s aren't much bigger because...well...those bikes aren't as big as the bikes that came in during the late 1970's through mid 1980's superbike era.
Still, most endurance racing tanks, and IOM TT racing tanks are usually closer to 5 US gallons (which is huge, in case you are used to those imperial gallons that are really just 3 pint glasses stacked on top of each other), so I feel like you aren't looking that hard.
Let's start with some basic facts: nothing is going to be "bolt on" for your bike. Well...maybe not nothing, but very little and certainly not tanks and seats. You are going to have to modify anything you get to fit.
your pre diversion XJ900 has a lot of DNA from the era of superbike racing, - you aren't going to make it look like a 1960's brit twin very easily, so you should forget that hot mess, and focus more on what road racing looked like in that time period.
https://commons.wikimedia.org/wiki/File:Yamaha_XJ-900_1000cc_24H_Montjuic_1983.JPG
There are lots of tank suppliers in the US, and some of them do ship overseas to the UK. I have to imagine given how prevalent racing is there that there are also tank and bodywork suppliers in the UK, but I'm not going to do that research for you, just make sure you are searching for racing tanks.
https://airtech-streamlining.com/vintage-fairings-seats-fenders-parts/vintagetanks.htm
Tannermatic
Home of Glass from the Past
FWIW, the two XJ "scramblers" that I have seen on the internet that I think are pretty used 1970's kawasaki kz650 tanks. I can't tell you how much work was needed to make it fit, just that that is what the spec sheet said it was. From what I remember of the KZ650, the tunnel is pretty wide under them. They also used custom seats and had extensive metalwork to the frame which sounds like is our of your league.
https://thebikeshed.cc/tcmc-xj900/
http://www.inazumacafe.com/2014/10/xj900-by-cafe-twin.html
Personally, I kinda like this more:
http://i.ebayimg.com/00/s/NDgwWDY0MA==/z/7mIAAOSwDYdcDX58/$_86.JPG
It looks like a stock diversion, but with a round headlight, some really low clipons, and some paint work and a reshaped stock seat. If I could change one thing it's to put rearsets on it but really...build this.
seriously...copy this guy (except for the shitty pod filters)