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.