Re: nissan 40hp plus 2000year
OB manufacturers build motors based on "families" of blocks. Breathing and timing typically make the differences between horsepower ratings within a family of motors.
For example, the current 4-stroke 2.5 and 3.5 have the same block, and the 4/5/6 have the same block; likewise, the 8/9.8 are based on the same block, etc., etc. The most famous of these in the old 2-stroke world was the 9.9/15. Lots of power difference (in percentage) by changing how the motor breathes.
Again, the specifics of your 40-to-50 differences will depend on the precise model you have. The Nissan model should be something like NS40C or similar. Without knowing your actual model, we would only be guessing.