Re: 1987 Mercury 70 hp trim/tilt Rebuild or Replace or Upgrade
Ditto the above - if you have any mechanical inclination you could rebuild it yourself. I expect a dealer would bill at least two hours for this so if you go that route you can figure on upwards of 300 bucks for parts/labour.
Would I personally upgrade to the jackplate with T/T - not on a 70 horse. A plate will allow you to "tweak" both the top speed and drag (gas mileage) of the motor, but when I say "tweak" I'm talking about fairly small numbers - definitely not the impression that the glossy ads would have you believe.
I use an adjustable jack (6" setback) on my 140 and by fiddling around with the adjustments and checking the results with a fuel flow meter I was able to save .6 gph (couldn't detect a noticeable gain in speed - as the "test day" progressed the speed varied by maybe 1 mph on the gps when testing for optimal gph at cruise speed). Since most boats see far less than even 50 hours use a year it would take quite a few years before you recover the 650 dollar list price of the unit you note.
If you want to fiddle with a jack, get a basic, manually adjustable unit (widely available for 150 bucks or less if you shop the discount places) - if you run the motor alot (like 100 hours a year), it will pay for itself in 2 or 3 seasons. If you have visions of getting more speed, then shop for a larger motor. If you squeeze 1 or 2 more mph out a jack (with a 70 horse) you would be doing very well.
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)