Yeah will usually get a bit of a voltage drop (maybe a drop of a couple of volts?) when the starter motor cranks over, but 6v is obviously too low. So you have a problem in the starter motor circuit between the battery and the solenoid.
According to the KE70 wiring diagram, there are only a couple of things in that circuit that can fail, they are:
- Fusible link AM1 (it will be one of the 3 wires that are crimped onto that big ring terminal at the positive battery post), just make sure it's not broken.
- The ignition barrel.
- The park safety switch (auto only)
- The starter motor relay (apparently some models have this? I don't think I've ever seen it).
- Wiring. It often gets brittle and breaks.
I'd suggest you get a wiring diagram so you know what you're doing, and start at the battery end of the circuit, work your way through the circuit with a multimeter testing the voltage at the above points of interest in the circuit until you find where you are getting a voltage drop.
Note: this is all assuming that there is nothing wrong with your battery or earth points, you should probably check those first because they are super easy.
Here's the part of the diagram you want: