If you are leaning towards trying the JB Weld...
For this job, it's gotta be JB Marine Weld. I'll leave an Amazon link to it below to show the exact one. All are not the same. Note the bond strength of 5020 psi, no other comes close. When applied and left to set, allow no less than 24 hours. It is NOT fast set. The fast set is much, much less in strength. For your job, I recommend at least 2 packages to insure you have enough.
Wash and clean the work area all around the break., including the broke off piece. When clean, wash it again with Dawn Dishwashing liquid , water, and clean away any chemicals. I recommend a wire wheel of a bench grinder for the broke off piece, dress the broke off edge flat on of the piece, then both sides of the break for at least an inch or a little more back on both sides to give the JB Weld a good bite on it. Then address the break on the lower unit head on to the break, and on both sides at least an inch for the JB Weld. A Dremel tool with a small wire wheel is ideal here, or perhaps a wire brush, or coarse grit sand paper. That paint is tougher than you think, and needs more than fine surface scratches. Get tough with it. This will be the one of the "Majors" that will decide if this works or not, along with proper alignment!
Be sure the broke off piece is flat and fits well. If not, consider a good hammer and anvil, or similar to persuade it. Consider a paint stir stick attached with a couple of clothes pins as a "Rest" under the broke off piece to hold it in place for the epoxy repair. When you are happy with the alignment of the two pieces and the "Rest" to hold it, Lift out the broke piece, mix some JB Weld 50/50, and apply it only to the broke edge of the piece in it's entirety (AKA "The Buttered Joint"), put the piece back on the rest and slide them together.
You will need some "Plastic Spreaders" of different sizes. An old credit card or expired gift card works great. Smooth the joint somewhat on each side while still moldable. This is only the beginning, just smooth out the high spots. Double check the alignment. You can move it a little for the first 5 minutes or so, but after that allow to cure AT LEAST 24 hours.
Second coat, next day - lightly sand both sides, clean, mix and apply more JB Weld. SPREAD IT SMOOTH. Apply small gobs here and there, spread with your plastic "Spreaders". Just do one side now, top or bottom. A little thicker over the broke areas, taper down a little into the wire wheel etched areas that extend an inch or so from it. It is not perfectly flat, there is a small tapered hump there, this gives it strength and allows the water to smoothly go over it without obstruction, not much of a hump, but only you will know. Smooth over everything you did before it fully dries, now allow another 24 hours to cure.
Next day, same on the other side, and include edges where they connect if needed. Smooth all you can with the spreaders before it sets, that stuff sets really hard! Look it over good to see if you need to spread a little here and there to smooth out anything. Allow to cure.
Next day, check your work. Should not look messy like a dirt dobber nest. Sand well, wipe off, spray paint with the closest match you can find.
This worked wonders for me. You might even remove your lower unit to do this work easier on a bench, and change your water pump while you are right there at it. I also preferred this way rather that expose all those lower seals to the heat of the other ways of doing the repairs.
Amazon.com