For the engine Serial Number you need to be looking either for a stamped data plate on the LH side of the cylinder block just forward of the flywheel housing. This plate will contain details of the engine model, the Serial Number, & the Arrangement Number.
Alternatively look for a stick-on film on top of the valve cover. That will contain the same information as the plate on the block but additionally will have information regarding engine power, fuel settings, max. operating altitude, etc, etc.
The Serial Number in total is eight characters. The first three can be letters or a combination of letters and digits. This is known as the Serial Number prefix which identifies the engine "family". Examples of the prefix for different models of 3126 engine would be 1WM, BKD, or G3E.
Each of the last five characters will be digits (0-9) and is the engine Serial Number within the family.