Maybe you just need some grease in the PTO spline to lube the spline and push the shaft fully back into the impeller spline till that bumper bottoms in the spline. Stop pumping grease once the shaft stops moving aftward or doesn't move after several pumps and/or if you see the PTO/shaft grease boot expand, otherwise it could burst like a baloon from too much internal grease pressure.
The black plastic collar on the drive shaft just in front of the impeller is supposed to keep the grease inside the impeller splines and keep the sand/dirt out of the impeller splines. If you replaced it the new one might be shorter than the original?
When I pump grease into my PTO spline, I usually can see the drive shaft move aftward slightly, sometimes almost 1/4" but usually a little less. I pump a few pumps into the PTO spline about twice per season and I stop pumping immediately when the drive shaft stops moving aftward toward the impeller.
So maybe in this case your PTO spline just needs a little grease to push the drive shaft aftward into the impeller spline?
A few things are important I think in this case, hopefully the splines should be fully engaged (drive shaft isn't too far forward or backward) and the splines need to have grease lubricant. It seems pumping some small amount of grease into my PTO spline tends to push the drive shaft aftward toward the impeller, this tells me the PTO spline was not full. If the drive shaft no longer moves aftward while pumping grease into the PTO spline, then I assume the spline is full and the drive shaft is fully seated into the impeller splines. This is the goal.
The purpose of the bumpers is to keep the drive shaft from moving too far in either direction and provide some cushion so the shaft isn't jammed too far into one end of the female spline where the end of the shaft can bind on the metallic surfaces, and the splines remain fully engaged along their length. For instance, if the drive shaft moves too far forward the splines inside the PTO might only be partially engaged, thus the coupling is weaker and might be prone to stripping the partially engaged splines.
So if your impeller bumper is in place, I think you can just pump some grease into the PTO until the drive shaft stops moving aftward. That's as far as the shaft will go b/c it's fully seated into the impeller at that point and both ends of the shaft splines should be fully engaged lengthwise. Then try adding a slight amount more following the first hour or so of operation, using the same criteria (drive shaft stops or won't move aftward). Look at the male splines, none should be exposed outside the female splines.
Explanation:
If the drive shaft moves too far forward into the PTO, or backward into the impeller (maybe missing rubber bumpers allows this), the splines might not be fully engaged and thus the strength of the spline coupling will be less able to handle engine torque than intended (splines could strip near the end of internal spline due to partial engagement).
So, your PTO spline needs grease occasionally to avoid running dry splines and increasing rate of wear.
Can you move the plastic impeller collar back and forth on the shaft away from the impeller, or is it well inserted against the impeller to keep the grease from washing out? That's why it's there, hopefully the grease in the impeller spline doesn't all wash away or become contaminated with abrasive sand.
drive shaft splines will wear from lack of grease or if the splines aren't fully engaged (location of shaft is incorrect), the black plastic collar on the shaft at the impeller helps to keep the grease in the impeller splines and dirt out.
So I dunno if there's a problem necessarily, maybe some additional grease in the PTO spline zert might help shove the shaft rearward into the impeller spline and close the gap you noticed between the plastic impeller collar and flange on the shaft.
I cannot move my shaft back and forth, the grease in the splines makes this difficult due to viscosity and hydraulic pressure. The shaft does typically move aftward while pumping some small volume of grease into the PTO, until the shaft rubber bumper bottoms into the impeller and stops moving.