I already wrote about my trials with impeller replacement on my Tohatsu. At the same time I was also doing my brother's 9.9 Honda. I was pretty sure that it would go smoother. I think that the Hondas are just designed better.
None the less, I found a way to make it difficult. Yup, I ordered cheap parts from Amazon. It looked like the pump would be easy to reassemble, but I couldn't come close to getting the impeller to slide over the key. I couldn't even get it to happen when I just tried to slide it down without the rest of the pump. Finally I pulled out the original impeller and it all went right together.
OK, the OEM parts came today and I had the pump reassembled and the whole lower unit slid back into the power head and fastened down in about 15 minutes.
Live and learn.
Keys can be very tricky. You probably could have used the original key and everything would have been fine. Keep the non-OEM parts for an emergency. And measure that key against the old OEM key by placing them on top of each other. Include thickness in this comparison. In most probability the difference is going to be very minute. But minute matters in this circumstance. Even a tiny rough spot will cause problems. You can shape the "new" key to the old ones shape by gentle sanding or with a very good file. The old Nissan 3.5 had a BALL for a key. About 1/2 the size of a BB. Man, THAT was tricky to get in right!
Actually, the key was fine. It didn't matter whether I used the amazon key or the original. It was the impeller itself. Its donut hole was too narrow and the slot was too small. It fit very snuggly on the shaft, and the slot simply wouldn't accept the key. I actually took a miniature file and tried to enlarge it, but that didn't work. It was way out of tolerance. I sent that kit back to Amazon and now I'll buy an OEM impeller for a spare. All good.
Over the years on cars, boats, and motorcycles I have bought OEM and money savers. Dumb bolt-ons mostly worked but parts that had to fit only paid off, particularly over time, as OEMs.
About OEM and "exact" replacement: Sometimes off brand replacement are close to or within tolerances specified by manufacturer, but not good enough. Found out when my auto mechanic tried to save me some money on a repair. Within tolerance was not good enough.