I started to do all this myself a couple of years ago, but other projects have gradually pushed it onto the back burner.
What I did was source a reasonably large industrial gear motor with a high reduction gear ratio, these come up on e-bay all the time.
But if you are patient, you will be able to get one one fairly cheap. This one set me back $26.00
That will give you a super strong cast iron gearbox casing, a massive support bearing and a pretty stout shaft, on which to hang your counterbalanced solar panel array.
Toss the original drive motor away, but keep the toothed plastic motor coupling.
Get yourself a stepper motor with another high ratio reduction gearbox and mount it on the back of the big gearbox using a flat plate, and the original toothed motor coupling.
Its mechanically fairly simple to do, but very robust.
You then drive the stepper from a constant frequency source such as a crystal oscillator and a frequency divider.
Now the actual frequency is less important than the fact that it is always constant and never drifts.
So the big gearbox might ideally rotate 180 degrees in 12 hours, but that is not important.
Suppose it only rotates 168 degrees in 12 hours with the particular crystal, divider, stepper, and gear ratios that you end up having.
As long as you reverse the direction EXACTLY every 12 hours, it will track pretty well, it will be a few degrees short at either end of travel, but that will not matter. Its not a cumulative error.
In fact less than 180 degrees rotation is actually desirable, because the solar array needs to clear the mounting post. I may need to modify this to do that....
To get a pretty accurate 12 hour reversing signal, which is very important, all you need is one of those cheap on/off appliance timers.
It will already have a lithium backup battery, so its ideal. Just program it for forwards at 6am and reverse at 6pm.
It will track the sun for months without needing any correction.
Its a bit of effort to accumulate all the parts, but once you have those, the whole timed tracking system becomes very simple and very reliable.
Here is the original three phase motor on the left, the stepper motor and gearbox on the right enclosed in a PVC weatherproof housing, with the plastic shaft coupling.
Behind is the the big gearbox.
Stepper is 200 steps/revolution (at 3.125Hz =16 seconds/rev)
Stepper gearbox is 100:1( 26.67 minutes/rev)
Big gearbox is 54:1(24 hours per rev)
At the other end in another weatherproof PVC housing are two hall effect limit switches, just in case the timer fails to reverse the action.
This also automatically re synchronizes the system if stepper motor power fails.
Also can be seen the keyed drive flange which supports the weight of the solar array.
The big gearbox bolts to this thick wall galvanized steel 4" x 4" post. Its two metres tall and there is 1.5 metres below ground set into a lot of concrete.
The half inch rectangular plate on top is angled at 38 degrees which is the latitude for Melbourne.
That is as far as I have come with it so far, but it has been running in mock up, and it does track the sun amazingly well..