Re: just wondering!
Your question comes up all the time, and about all kinds of engines. The simple answer is no. You can, of course, go to the BRP site and compare parts numbers from your 20 hp and the 25 and 30 hp motors to see what's different, and I don't know those engines from that year.
But...the answer's still going to be that it is uneconomical to do what you're suggesting. Even if the engines use the same block and have the same displacement, other parts, like exhaust design, porting, reeds, manifolds, carburetors and more may be different.
The best answer is to say that if you want a 25 or 30 hp outboard, you should sell your currently well-running one and buy the one you want. Typically, the price difference will be far less than the cost of buying whatever parts would be needed to do the upgrade, if such an upgrade is even possible. In most cases, it is not possible.
