||One way to visualize the current flow through a wire is to think of a pipe and flowing water. The thicker the wire the larger the pipe and the easier it is to flow water/current through it. A small diameter pipe flows less water/current etc. Probably everyone is up to at least this point in understanding wire/electricity.
But the Ohms law shows that as you have less voltage to work with you need more current to flow to get the same amount of work done. The amount of wattage the starter motor needs to do its work varies by bike so let's use the work we need to get done to run both highbeams which are pretty much standard across all bikes.
2 X 65 watts = 130 watts
to get 130 watts using a 12 volt system (which is calculated at 13.2 volts) we need 9.85 amps of current flow
to get 130 watts using a 120 volt system we need 1.85 amps of current flow
to get 130 watts using a 240 volt system we need .54 amps of current flow
As wire ages it flows less current which is why all motorcycles and cars have problems with their electrical systems. And it is why many new cars are now starting to come with 24 volt (and even higher) voltage systems.
One way to visualize the problem is to think of a tree and the way that a tree grows larger by adding a ring around the outside each year. With wire and current flow the opposite happens, the wire adds "impediments to current flow" over time. If you think of adding a ring of impediment to the *inside* diameter of the pipe you can see that as time goes on the actual size of the pipe we have to flow current through gets smaller and smaller.
I used the term "impediments to current flow" instead of "resistance" because it turns out you can't really measure much resistance in a wire that has lost a lot of its ability to flow current. On a normal digital voltmeter you probably won't measure any difference in ohms. We know because we tried. We did find a site once that detailed the very small increase in measurable resistance that would bring about a marked reduction in current flow but have lost the URL.
If you do want to measure the losses in old wire you can measure the voltage at the battery and then at the end where you want to get the work done, in the case of starting a motorcycle, that means at the wire that goes on the terminal at the starter. When we have measured the voltage available at the headlights on various bikes it was about 11 volts when the battery had 12.85 for a 15% loss. Because the headlamp output drops at a different rate than the voltage loss, the loss in visible light output is more like 44%. We would assume the reduction in work accomplished is similar for a starter motor.
One of the main reasons for the degradation in the ability to flow current is that when the wire is made impurities (oxygen, dust, whatever else is around) are mixed in with the copper which over time oxidize changing the characteristics of the wire and reduce its ability to flow current. That is what our inner ring example was trying to help you imagine. This degradation is along the length of the wire and not just at the ends where you see some corrosion on the terminals.
The specialty (high end) audio business became aware of this many years ago and there are a wide range of wires now that are specially made to have very low levels of contaminants. At one time some companies even claimed 100% oxygen-free-copper but have reduced it to 99.99% oxygen-free-copper to keep the lawyers at bay. In other words, you are getting nearly pure copper with virtually no contaminants to accelerate its degradation over time.
These highly specialized wires flow much more current at low voltages than conventional wire and much more than marine grade wire with its corrosion resistant plating. If you understand that copper flows current better than anything other than silver (copper even flows current better than gold), and that the tin plating of marine wire is to cover up the copper so you don't see the corrosion on the copper, you can see that for 12V systems marine wire is the absolute worst way to go. It is a bandaid on the corrosion problem rather than a solution to the problem.
On some bikes (like the Ducati E900) the starter circuit is about 9 feet long. Plus the current has to flow through the engine cases to the ground lead and then to the battery so about 11 feet total. That is over double the length of a Monster or 900SS circuit. So the losses along this length are double of the other models of Ducati and yet after 6+ years in the field our original HICAP circuits are still enabling E900 bikes to start in 1 second.
A new outboarded circuit is all that is needed to make a difficult to start bike start better than new (the wire was probably a year or so old before they even made up the OEM looms so it had already begun to degrade). On most bikes A HICAP is a 30-90 minute install.
There really is no need to replace the brushes or starter motors unless they have been seriously degraded due to riding through salt spray and other conditions that would degrade them more than the normal usage.
On the 1975 Norton MK3 they have a starting system which was notorious for not being able to start the bike.
The options were;
buy a new battery every 6 months,
rebuild the starter with a $200 (4) brush kit,
buy a new upgraded starter for $395.
The HICAP starter circuit for that bike costs $75 and after a 45 minute install the bike will start as well as with any of the other solutions. And will continue to start the bike for many many years.