Before buying any kits, I would remove the carburetors from the motorcycle & clean everything under the float bowls. Check the float needle valves & seats for wear or pitting. Mine were fine, but I went ahead & polished them with bronze wool. The brass parts were okay as well, so I just replaced o-rings, gaskets & rubber stuff. Carb kits can be a waste of money as you may pay for extra unneeded parts & some brass parts aren't correct.
The fuel valve may need a rebuild kit or replacing. Check Partzilla to see what's available.
Z1Enterprises dot come may have some stuff for your motorcycle.
Engine oil. I use either Shell Rotella T 15W-40 regular in the white bottle or Chevron Delo 15W-40 regular. Both of these are heavy duty oils marketed for diesel engine use, but have the anti-wear additives & they usually go on sale at various places. Shell Rotella has the JASO-MA rating. Not sure about Chevron Delo, but it has pretty much the same anti-wear stuff as the Shell product.
1981 KZ650-CSR. Competition, Sports & Road.