Re: removing lower unit
I would use the Yamaha parts for sure, the entire water pump kit is only about $50. A pair of 2"X4" boards, side by side, and about 4"apart, screwed into the top of your workbench, hanging over the front about 15" makes a great place to set your lower unit to work on it.
The 90HP Yamaha doesn't have those little wave washer/compression rings, so that's just one less thing to deal with. Make sure your impellar vanes are turned in the appropriate direction for driveshaft rotation(make a mental note when you first look at it). Not that much to it, as long as none of your bolts don't break off due to corrosion. I used the Quicksilver 2-4-C Waterproof grease for impellar re-assembly and all bolt threads. Good Luck!