That looks like the original cover is still around the shaft but has broken lose? Personally, I think I'd just cut the remaining old cover off and not replace it.
The reason is, they're a liability b/c many times when they break they are digested into the impeller and they take up some space in the tunnel that could be occupied by water instead, performance might be slightly better.
The benefit of the drive shaft cover is it might save your shaft from being bent in case you run over a rope and suck it into the pump.
My boat no longer has it's drive shaft cover and I've sucked my rope into the pump once with no damage to the shaft. I guess it's gonna happen at some point though, despite I've been driving jets for years I've sucked up my rope only a few times due to being careless.