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will a new 90 horse four stroke be enough motor for a 1987 montauk and will it be to heavey

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Honda had the lightest 90 hp 2 years ago. I would check, this may have changed.
Montauks were powered with the original honda 90s and that was a lot of weight.
Looked heavy.
Good Luck.

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The 17' Classic hull has a max hp rating of 100.

As no one makes a 100hp these days, most people go for the 90hp or even a 70hp if they are not into speed.
Take a look through the members personal page section at the 16' and 17' Classic models.
http://www.whalercentral.com/viewpage...page_id=65

This list was last update in June of 2015
http://www.whalercentral.com/articles...ticle_id=5

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I have an ETEC 90 on my 1988 Montauk and love it. I believe that Evinrude is offering a 10 year factory backed warranty on ETEC's right now.

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Honda offers a reasonably lightweight 100 HP fourstroke engine:

http://marine.honda.com/outboards/mot...tail/BF100

But keep in mind that Honda's 1.5 liter 100 HP engine is the same weight as Mercury's 2.1L Fourstroke 90 HP engine (and Mercury's 2.1L 115 HP engine as well).

With the same weight and 40% more displacement, the Merc looks like the more attractive option. But I have not priced that Honda 100 HP engine, so it may be a good choice for a Montauk especially if there is a price advantage.

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Yamaha has a new 90, 1.8L weighing in at 353lbs (if my math is right)

http://2017newproducts.yamahaoutboard...f90-75.php

It should be an awesome motor.

I have an F70 and love it.

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The Suzuki DF90 is the lightest four stroke at 343lbs

And now the new 2017 Yamaha F90 is the second lightest four stroke at 353lbs (if my math is right - they say the new model is 13lbs lighter)

The Honda BF90 four stroke is at 359lbs

http://www.suzukimarine.com/Product%2...DF90A.aspx

http://2017newproducts.yamahaoutboard...f90-75.php

http://marine.honda.com/outboards/mot...il/BF75-90