[tsubaki]

The plastic tube is incorrectly placed and probably the whole problem, done gone this far might as well replace it all, weather the impeller is bad or not. Most likely the last person that was in there (thought he) aligned it and tightened the bolts up to pull everything together.
The shifter in a given location is to be able to loosen the fitting holding the shift rod and/or a threaded pin usually located front center of the powerhead, below the carbs intake.
Do not rotate the shift rod once disconnected from the powerhead, this is the threaded adjustment.
The new pump or kit may come with a couple of adaptors for different tube sizes.
Installing the new impeller and aligning the key is easier done by rotating the prop or shaft and sliding the housing in place at the same time. Find some long shouldered bolts, cut the head off, thread into the foot where the ones for the pump goes and put it together.

When reassembling you may have to rotate the prop to align the splines of the shaft to the powerhead and slightly engage or disengage the shifter to realign to tighten. Sealers (if required) like Permatex will be fine.
Don***8217;t be afraid to remove the foot and recheck tube alignment before finishing bolting together, better to feel comfortable instead of wondering about it.
Reassemble with Antiseize or the like.