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The Yamaha 2-stroke 90 and the Evinrude E-tec 90 are very similar in actual HP at the prop, and should run neck and neck. They do not use the same prop, however, since they have different gear ratios. Both usually give 40 MPH, maybe a little better, on a classic Montauk. It should also be noted that the prior 16' hulls are lighter and run faster with the same HP, so they are not a good comparison.

Both the Yamaha 90 and Mercury 90 2-strokes use a 2.33 gear ratio. When Whaler was selling these boats factory rigged with Mercury 90 2-strokes, the furnished prop was either a Mercury Black Max aluminum 21" pitch, or a Laser II 20" pitch. So the Yamaha maybe should be able to turn a 19" pitch prop at least, since I do believe the Merc puts out more power than the Yamaha. I don't think the Yamaha 90 can turn a 21" pitch prop like the Merc does.

With the engine raised up, and putting out the rated HP, the fastest prop you can buy for that rig would be a Mercury Laser II in 19" pitch. They give tremendous hull lift and top speed on a Montauk, and are faster than a Stiletto on an actual pitch for pitch basis. If your engine can't turn anybody's 19" prop raised up (or a 17" Stiletto), the engine should be looked at. Sometimes used engines can look nice cosmetically, but are worn out internally.

You appear to be only getting 70 HP out of it.