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The classic Outrage 21 models, 1971-1979, can easily handle 600# or more on the transom.
My twins weight 610# combined.

http://photobucket.com/gallery/user/l...Tc=/?ref=1

Since it appears you are looking for a big single, you want to avoid the huge looking engines.
They just don't look right on these classic 21's. I think the absolute best option in new 4-stroke power would be the Mercury 150. It has the most displacement of any of them, the smallest profile, the lightest weight and the most power - probably around 165HP. They are also competitively priced, around $10,600. And this engine is still large looking.

If you check the Metan classic reproduction site, I think there is a photo of one their 21 Smoothside knockoffs with one of these Merc 150s on it. You can see what it would look like on yours.

The worst mistake you could make for a classic 21 would be to select an Evinrude G-2 - they are monstrosities both in size and looks in my opinion, and just don't fit with any classic Whaler.