The difference between the 2 motors is the carb gasket, and the timing at WOT.
The gasket on the 25 is slightly smaller. You can either buy the 30 hp replacement gasket, or if the old one is still in good condition, you can cut it. The timing is advanced by 5 degrees.
Attached is the timing, if you need i can also put up a picture of what the new gasket looks like compared to the old.
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