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There is such a problem: the starter relay clicks, but the starter does not crank. Most likely, the issue lies with the 'pennies'.
This engine has great access to the starter from above; it's held by only two bolts.

I clamp it in a vise and disassemble it. All starters are almost identical; this one is old and without a reducer but otherwise similar to any YouTube video example.

Two long bolts that tighten the entire housing and two short ones that hold the brushes.

The relay hasn't seized, but I still polished the piston with zero-grit sandpaper to remove all rust and dirt. Now it moves very smoothly.


Next, I worked on the electric motor. Used contact cleaner, a cloth, and a needle for cleaning the rotor.


To prevent the brushes from falling out, I left a head of appropriate size.

At the point where the brushes touch, I carefully removed graphite debris with a needle.
I reassemble it. Beforehand, I polished the shaft and bushings (there are no bearings) and heavily lubricated them.
Reassembled, I decided to check it out. Connecting wires for jumper cables and closing the circuit, everything works perfectly.

So happy, I put it back in place, turn the key, but nothing happens. Tried with a screwdriver, it works.
Went home to eat and think. In theory, I knew how the starter worked, but this was my first time working on one. A bit of googling revealed that these were 'pennies'. Later in the chat, my guess was confirmed) Alright, continuing. Removed the starter again and thought about how to spread it.

After futile attempts with screwdrivers, sharp rods, and other tools, I decided to cut it. Not in half, but carefully made cuts with a hand drill and small cutoff wheel.

Now it's easy to bend open

Then it's simple; heated the solder well with a soldering iron so that wires come out and the cover comes off easily)

And here is the reason for all problems

'Penny' extremely rusty, enough without load, but under load it no longer works. I clamped it in a screwdriver and polished it with sandpaper for 10 seconds)

I also cleaned the counterpart (bolt heads) but didn't take photos. And reassembled everything in reverse order. If there is wear, you can change bolts or just rotate them by 180.

By the way, decided to repaint it. Basically pointless as it's very dirty and oil leaks, but I thought I'd give it a quick spray anyway)

Put it back and it works right away!