I have used the OMC/BRP 277181 impeller with good success. You must "thin it", though, to make it work. The side profile of the Fleetwin impeller is smaller, so I took the new impeller and thinned it on a belt sander. Flip frequently to take equal amounts off both sides until the profile dimension is the same as the one you removed. Take care to keep the impeller flat on the sander as you don’t want it uneven, either… My ’53 pumped great with its new impeller after this modification!

No, you don’t have to remove the powerhead for the neutral clutch models. Drop the unit and undo the shift linkage from under the exhaust snout.

Note that everything is "keyed" to keep it in proper alignment. And the flat spring under it is the only thing keeping it pressed together.
Unusual, but what the hey, they worked. At least you won’t be busting off pump housing screws.

I also have seal 202274 for that motor,if you need it Stan see above