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The Max HP for those years were 150.

Many Outrage 18' from those days had twin 70hp on it.
Here is one of my prior Outrage 18's with twin 70hp.
http://users.sisqtel.net/~jkriz/Outra...age86.html
Evinrude 70hp = 248lbs each
plus 2 batteries
plus 2 accumix oiler
We are looking at around 600 pounds total for approved hp.

There are other that may have more weight but they exceed the Maximum HP.
As you mentions, some have twin 90hp and some others have twind 115hp.
Unfortunately they obviously exceed what Whaler recommended in hp.

I would say you are good to go with any 150hp today that doesn't exceed a total of 600 pounds as the twin 70's above.
Of course the lighter the better.
http://www.whalercentral.com/articles...ticle_id=6