Have 25 hp ES 2 stroke Outboard Engine, Belgian Made, 2004
It has blown.
Have seen a 20 hp similar outboard engine 2000, I am told that this engine is identical to mine
Save a bigger Carburetor which makes it 25 hp.

Question would carburettor and electrics from 25 hp upgrade the 20 hp ?

It is doubtful. Horsepower is achieve by more then just a few parts on the outside. It may help the smaller engine or it may overwhelm it.

The best way to check this is to go to a Mercury parts diagram. Look up both engines by the serial number. You can find the diagrams here Parts Diagrams Application Charts Yamaha Mercury Johnson Evinrude Mercruiser Seadoo - PartsPak.com (http://www.partspak.com/productcart/pc/viewContent.asp?idpage=13)
Compare both sets of engine parts and then decide if the majority of the parts (ie engine block, etc) are the same.