I finally did it. I changed the carburetor and adaptor plate(w/ exhaust tube). Got a 20% increase in top end speed. Now I just have to change the decals. If you do it too, make sure you buy a new adapter plate gasket and powerhead gasket because mine were fused to the metal. I recommend Mercuryperformance.com for their price and customer service.
Do your research The ob builders often use the same block for a family of motors but often the porting in the block is different Compare the part number for the block as well. When I sold suzuki motors Their 3 cyl 25 hp motor was excatly the same as the 30hp but with a throttle stop on the carbs to limit throttle opening