Seal the edges of the decking with resin. That will be fine.
Drill and fill the screw holes with silicone caulk, before installing the SS screws.
The plastic hatch will allow walking on it, until the sun weakens it.
When you take the gearcase off, check where the shift rod goes thru the shift cover. Those covers can corrode and swell grabbing the shift rod tightly, causing the shift linkage to give and not allowing the motor to shift into forward easily.
If the insulation is still good, you could recover it with new plastic to keep the pieces contained. use some heavy trash bags cut to fit and some spray on adhesive.
Alternatively, there is an insulation which looks like silver bubble wrap. It would be fastened with the same spray on...
As long as you detect a signal, you are probably good, as they either work or don't.
Not sure you can see a half sine wave on a voltmeter. Try the AC setting on the voltmeter and step it down in voltage until you get something.
A 1975 Merc 850 would have a distributor and battery CDI ignition. A 1976 Merc 850 would have ADI ignition, as you describe. A small point.
See if there is a test for your ignition components on the CDI Electronics web site.
Wait, someone said "When in doubt, trim it out"
Trim it until the steering feels light. A lot of cruisers, as you have noted, do not benefit from trim. Your boat, depending on hull style, may benefit a lot or a little from trim.
Carbs closed do not control the idle speed. Idle timing controls the idle speed. So back off on the idle screw and adjust the throttle cable to push the throttle closed (retarding idle timing) to control the idle speed.
It clearly has a bad miss. Likely on one cylinder. You should inspect the spark plugs and see if you can figure out which cylinder. Maybe an idle adjustment is in order?
You can use an ohmmeter to verity the points open at the right time. Remove the point wire from the coil. Set the ohmmeter to read continuity, and connect it to the point wire and ground. turn the motor over and see that the points open (continuity is zero) when the timing marks on the...
Those are a very good design. Sorry you are having issues.
You should probably get an OEM service manual, and go thru it, as a new motor is likely 15K. Since you did carbs, check compression, spark on all cylinders and then link and synch.
Watch for cracks on the top corners of the transom, where it meets the gunwales. When you see those, or other deformation on the transom, you are close to transom failure. Other than that, it sounds like you have some time.
You should see a wave coming off the hull at least 3/4 of the way back from the bow. if not the bow is plowing water. Can you post a picture of the hull defects. A hook in the transom area will cause the issue.
Trim the motor up and see if the bow lightens. If not, you may need to raise the...
Adjust your throttle cable barrell nut to push the throttle closed. That will lower the idle speed. I would recommend a 21* BTDC max spark advance, if you plan to use regular grade gasoline.
Likely the second set of points was not set correctly. My fat fifty did that same thing. I was able to remove the crush washer and there was enough threads to hold the spark plug in.
A solid steel spark plug insert is another option. You would simply need a new head gasket and the insert.
