9.9hp Powerhead Base Gaskets

I picked up an early 2000's Yamaha 9.9 4-stroke from an older gentleman. He indicated that it leaked engine oil out the exhaust badly after he attempted to replaced a cracked oil sump, and the aluminum plate that goes between the sump and powerhead. Everything looked to be assembled neatly, so I changed oil, fired it up in the barrel, and sure enough, smokey exhaust, and rapid brown creamy oil buildup in the barrel.
I've now pulled the powerhead, and this is what I see.

It looks like oil/water have been mixing. Before I go any further, I thought I'd check to see if anyone can offer guidance on the attempted repair process. I have a big box of gaskets that were included, many new OEM parts. I'm wondering if he forgot a gasket on assembly last time around? Is this oil sump cracking a common failure on these motors? Might there be some repair discusions documented somewhere? I've done some searching with no luck so far.

Any thoughts or guidance would be appreciated. Failing that, I'll stumble ahead with tools and shop manual at hand, and see what I can figure out!

Update: I pulled the sump out, and started the cleaning and inspection of the parts. I don't see any gaskets missing, BUT, the oil pump relief valve was seized in the closed position. I needed to remove it, mount it in the vice, and give it a rap with flat punch to break the spool loose. Once broken loose I worked it back and force with solvent, and now it has full travel without any signs of sticking.
So...what are the odds that was the issue, and careful re-sealing of the sump and adapter plate will solve the problem? Are there other lubrication system components I should be checking while the powerhead is off?