I’m restoring a wrecked ‘85 (now with a ‘84 frame) K100RS cafe racer. The tank is badly dented and I am thinking of replacing it with something smaller and more cafe racery. Is this doable? The original tank has two fuel hoses, which I suspect is the fuel supply and return, which most other tanks don’t have. Is there a work around?

I believe others have gone down this path how successful they were I don't know? Anything is possible, there are external inline fuel injection pumps available online try Ebay.  The BMW style pump is available as an inline pump but as the immersion in fuel is supposed to cool the OEM pump  I don't know how hot the inline one will get. I'd be cautious as you don't want an external pump spewing fuel. A suitable tank that fits might be long hunt. Take measurements and see your local friendly motorcycle wrecker and see if he'll let you ferret around in his old tanks. A return line spigot will have to be added to the substitute tank more likely than not, but is doable. 
Regards Martin.

Some Moto Guzzis from 1990 onwards use an external fuel pump and filter (e.g. my 1993 Cali 1100ie). The only instruction in the manual for the pump is 'do not run them if there is no fuel in the tank' as the fuel is used to cool the pump.

The tank might be tricky. The K's frame around the headstock is very unlike any other bike, mainly because of the width of the top rails. Any smallish tank or tank for a traditional frame or single top tube just won't come close. I'm still looking for a tank that will give a K a (sort of) flat tracker look.
I did wonder if a tank from a trellis framed ducati might be something like though?
If you make any progress, I'd be interested to learn.

As for the fuel pump, there's two main types; 'in-tank' which tend to be smaller and require immersion in order to stay cool, and external, which don't but are physically larger (about the same size as the OE 'in-tank' one, funnily enough) Both types can be found very cheaply on Oohbay.

Edit: I forgot, in my past efforts to maybe find an alternative tank, I did end up considering a fibreglass dummy tank (a sort of shell that sits over a utilitarian welded up ally 'box' tank) of the sort sold by Complete Cafe Racer. Being a shell, there'd be a lot more possibility for cutting and carving to make it fit, whilst the 'real' tank could be fabricated to fit inside the frame top tubes, being dog ugly as a consequence.

The angle of the tank can be changed by modifying the front tank mounts which can help with the aesthetics. Too many cafe's stick with the stock tank, and only a few have managed to overcome the conflicting angles satisfactorily. Consider buying a steel tank and having a new floor made/modified easier/cheaper to get welded than the OEM aluminium tank.
Regards Martin.

I think this bike is the best example I've seen of working with the insane collection of angles and lines that come with the OE tank. The builder has managed to replicate most of the lines of the tank somewhere else on the bike; a kind of 'hiding an elephant in a field of elephants' approach.

Clever paint schemes and stripes and custom side cover angles/seat angles as well as additions to the tank to change the lower angles can also help.
Regards Martin.

I like this build, which coyly references a fuel tank from an old Spanish 125. The tank is nicely hidden by the frame. So, it can be done, especially, as Martin suggest with welding on some adaptors (need to weld the return hose attachment anyway).

I think you can use any tank you want if you want to modify for a cafe racer. I think some Harley tanks are actually two tanks, so that might work. 

One option for the fuel return (although a return to the tank would be better) you could could tee the return upstream of the pump. I'm assuming the pump is external, so then the return would be between the tank and the pump.