Two Briggs engines same displacement, physical size, different HP.

I have the following two Briggs engines.
1) Model 445877
and
2) Model 407777
 
Both have the exact same displacement.
However 1) is listed as 24HP, while 2) is listed as 20HP.
 
Is it only a carburetor efficiency difference or is there severely different changes such as cylinder head shape difference which gives (1) 4 more hp ?
 
What i want to see is if I cannot make changes to get the 20HP (2)  to 24HP, so I can use it in the same riding mower as they are otherwise basically identical.