How to disassemble this engine

I want an explanation or a video on how to disassemble the parts of this engine because there is a problem with one of its parts and I want to replace the damaged part with another part

 

The only part that can be disassembled is the gearbox above the black ring. However, this won't help with the repair since the issue lies inside the motor itself. It's best not to waste your time—consider purchasing a new one instead.

 

Is it normal for the engine to feel heavy when turning by hand?

 

Yes, it's a geared motor and you are trying to turn it from gearbox. Apply 24VDC and see if it's turning.

 

How to know the engine revolutions if there is no data on it
Is this determinant in a red circle the number of engine revolutions per minute?

 

What is the problem with this engine?

 

I want to know the engine RPM because there is another engine but it doesn't have data to know its RPM I want to make it install instead of the old engine

 

In addition, when changing one gear set from another, there is a great deal of difficulty in connecting the gear set, but the old set is connected very easily.