Easiest and safest upgrade is the AGM or the sealed lead-acid ones from Yuasa. You need a battery made from virgin lead, and only these 2 types come that way today. Lithium-ion batteries are problematic, and have even burned these bikes down by catching on fire, so unless you are real savvy about getting the proper (and expensive) electronic voltage regulator to use with those, avoid them.
You can also install a field coil from the CB750A bike: this will increase the output of the alternator above 2200 RPM by about 20% more than the one you have now.
Another thing with the K1: the alternator bullet connectors in that bike were zinc-plated, 3.0mm size. They are usually cooked black by now, losing about 15% of the alternator power as heat. You can get new brass ones, slightly larger (like in the later 750K4+) at 3.5mm size, from Vintageconnections.com. Look up their 3.5mm Bullet connectors in male and female. These are right behind the sprocket cover, to your left as you look at the engine. While it takes some patience, you can change them out right there. You will have to trim off about 1/8" of the back side of the female silicone boots in order to make them fit, but it works. I do it to every engine that comes thru my shop. This will restore the lost power from the cooked plugs you have now.
