This is a great old outboard, and you are wise wanting to get that gear case resealed properly to avoid lube leaking out, and water leaking inside the gearcase. Running the engine with a gearcase full of water will eventually destroy many of the internal components, including the driveshaft bushings in the gear housing itself.
First, in order to pinpoint and evaluate leaks properly, the gearcase should be "pressure tested". The gearcase can then be submerged in water, you will see air bubbles where there is a sealing problem (the gearcase needs to be removed to do this test properly).
Working on one of these gearcases isn't particularly difficult, but sealing it up properly is a detailed process that can be difficult. Trying to do this yourself, with little experience, and no special tools, will not lead to success.
You mention that lube is leaking from the two forward screws securing the skeg to the gearcase. These screws do need to be sealed in order to avoid leaks. The screws may not need to be replaced, it depends on the condition of the sealing surface under the screw head. The aluminum mating surface on the skeg could be gouged/deformed as well. Have a look at these surfaces after draining the gearcase and removing the two forward screws. Using a rugged/hardening sealer (such as the OMC type M/847 sealer) under the screw heads is usually all that is needed to seal these areas sufficiently, this will seal the mating surface between the screw head and aluminum hsg. Please do not remove the little phillips screw next to the lower lube drain plug, this secures the inner shift linkage which will fall apart if the screw is removed! In order to refill the gearcase, both upper and lower lube screws must be removed, the lube is pumped in from the bottom until it flows out the top screw. The top screw is then reinstalled creating a vacuum inside the gearcase minimizing leakage while the lube filler is removed and the lower screw is reinstalled. Please make sure each lube drain/fill/vent screw has one and only one gasket. Mixing up this step will create another gearcase leak.
In short, I don't recommend you attempting to pull this gearcase apart and attempting to reseal it. But, you can surely remove those leaking screws and reseal/replace them though. From what you have written, I don't have a lot of confidence in the repair that was made by your retired friend. Needless to say, leaks like you have described would have easily shown up if a proper pressure test was performed after he completed the job.