Manual states it is a 1/8" diameter roll pin but does not say how long.
Needs to be correct length to stake both ends so it won't fall out.
Cat part # is 4H5232. May have to go to a larger size if the hole is hogged out.

Also suggests replacing the packing on that shaft if there is any sign of leakage. Part # 7H6986

I have the rear cover 8h2591 and have the long transmission rod and you should check your brake cluch and discs condition before reinstaled

Where about are you from and what kind of prices would you want? I have found some parts fairly local, but not cheap!

ok, Update with some pictures and questions! First would be oil, The engine Im sure just takes 10w30, what does the pup transmission take... same thing? 0w40...?

So this is where I am so far.

I have a question if anyone knows, about the roll pins. The bolt has two holes for roll pins, it looks like someone has messed with this before, but Im wondering if I could put this "top" roll pin in, where the roll pin could be wider than the shaft. My plan is to install both roll pins on the shaft where the bearing is, so hopefully this never happens again. This picture shows a roll pin loosely installed, and the other roll pin hole is a couple inches lower.

Picture of the latch (probably wrong terminology!)

Im wondering for curiosity sake, what this piece does. It has a valve looking deal that has a spring, and a bolt to put tension on the valve looking piece.

Those are some good pictures.
Diesel engine, starting engine crankcase and starting engine transmission should all be SAE 30.

Roll pin is to be installed at specific dimension measured 1.547" back from the rear of the bearing collar with 0.010 to 0.020" clearance between the pinion and spacer ( with pinion latch engaged).

Your "latch" is actually the pinion engagement lever.

That empty threaded hole is where the pinion latch should be. It is spring loaded and adjusted to a 3/16" height between the body and nut with the pinion latch engaged.
This is what latches the pinion in place and releases when centrifugal force reaches a certain rpm.

No reason you can't put an extra pin in there if you want to. Use the single wall type roll pin, not the wrapped type. They need to fit snug in the hole and be staked in place.

Thanks a ton for the explanation for how that all works Old Magnet! I do have a question about the packing, do you by any chance have a diagram or parts order in where exactly it goes?
I also figured out a huge problemt today that part of my issue is that the threads where the long bolt goes into, are no good, and the roll pin is actually kind of a backup to make sure this all doesnt come apart. So I am in need of ... Anyone happen to have a part number for this piece...? Its a nightmare calling up the cat dealer and trying to explain what part I need. So far they have ordered the wrong things for me a couple times now. They are helpful though, I will give them that!

Here's the parts diagram for your unit.
As far as the packing it takes a couple of different diameter shafts or long pin punches to remove and install.

The end femelle rod # is 5m6758 and am still have some
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No, the (female) threaded "Pinion Engaging Collar" is a 7H6982 according to the parts diagram.
The long threaded bolt (shaft) is a 4M6163.

5M6758 is the engagement plunger.

I make a mistake and OLD MAGNET is correct the collar femelle shaft is 7h6982 and have some