Hi Ian, i reckon you're overthinking the brass to aluminium corrosion thing a little. By the time that corrosion sets in that cover will have cracked in a different spot. It's common for the front right and the rear left wheel on most mowers to cop a beating due to the clockwise direction of the engine. Any debris(stones etc.) get thrown out in the direction of the spin. Once you press it back into the wheel run a squirt of super glue around the perimeter to prevent it from falling out. Once done switch the 2 front wheels around to preserve that one a little longer.
As for the blade boss, i've seen a few with one tang broken off that work fine, so that's your choice as to whether you replace it or live with it.
Genuine handle knob can be found
HERE
