Re: Johnson 33 help!?!....
Your background as an auto mechanic will serve you well in this project. Your motor is a very basic two-stroke motor and is relatively easy to work on.
You will also find that many of the parts for it are still available through your local OMC/BRP dealer. One of the reasons why this is true, is that the basic engine design was used for about 20 years, from the middle 50s through the middle seventies. Horsepower variations varied from 28hp to 40hp.
In addition to the items mentioned in JB's FAQ post, my advice is to start with a basic compression test. To me, this is the starting point of determining engine health. I would consider relatively even compressions (10% variation or less between the cylinders) of 90 to 110 psi to be acceptable on this motor. You might see higher numbers and the motor would probably run OK with lower numbers, but the range I am suggesting should result in an engine that performs pretty well. That assumes, of course, that there aren't other major engine issues.
After this, check the motor for obvious signs of overheating. Blackened paint on the powerhead is a near dead giveaway. Look for corrosion too - if your motor has "lived" in your area for its entire existence, chances are this won't be much of an issue.
The next thing on my list would be to drain the gearbox oil to check for presence of water and/or metal shavings. See JB's comments on some of the things to look for.
If this motor has an electric start, you already know if the starter is in serviceable condition, simply because you have used it.
All of the items that I have mentioned thus far are items that I believe to be the ones that will be generally the most expensive and/or difficult to deal with on a 1970 33hp OMC motor. If they all look pretty good, you enter the point at which going through the basic systems becomes the thing to do.
I usually start with fuel delivery or ignition. Rebuilding the carb is very easy and the rebuild kit should be readily available. You can also get a rebuild kit for the fuel pump. The pumps are kind of expensive for what they are, so this is a nice way to save a bit of money. The rebuild kits from OMC/BRP also come with a instruction sheet. On the ignition side, the motor is equally simple. It has a magneto system, which consists of the flywheel, a pair of coils, points and condensors. Nothing exotic in this motor.
The cooling system is about as simple as it gets. There is a drive shaft driven water pump in the lower unit, right above the gearbox. Once you drop the gearbox to gain access to the pump (attached to the GB), you can inspect the housing to determine if you need to replace just the impeller or the whole assembly. If you get the water pump rebuild kit, you will have a new housing, upper and lower water tube grommets, driveshaft key, and impeller.
Beyond these basic systems, there are a few issues to check and/or repair. Given your background, you could probably take the motor apart and figure out how to fix most of it just by general experience and intuitive mechanical sense. That said, having a service manual available will reveal information on a few items, such as the vacuum cutout switch that prevents engine over-run, and a few other items. You will also need the manual for lube info, specs, torque values, etc. Try to get an OEM service manual (possibly available right here at iboats) if you can - the aftermarket manuals aren't any better than they are in the automotive world.
One other thing that comes to mind is that you seem to have a "northern, fresh water motor." To me, those are the motors to get your hands on if you like old motors, because they don't have the corrosion problems that salt water motors have. They are also very often, relatively low hour motors due to short seasons.
You may just have your wife buying you dinner after you get this motor in shape! Once you do, you can also remind her that a new motor of similar horsepower would have costs thousands more!
