This is the impeller kit I use in my engine. It comes complete with gaskkets, instructions and impeller lube which is nothing more than glycerin.
If you notice, the box does not say Westerbeke on it. Westerbeke uses a Johnson pump on this particular engine and Hamilton Marine stocks the exact replacement kit for it. They may for your engine too.
The impeller on the left is the brand new one and the one on the right was removed last fall after winterizing the engine. I keep the old ones, that are in good condition, as extra, extra spares. Sure, I could buy this impeller through Westerbeke but it costs almost $38.00 vs. $16.00 at Hamilton Marine and they are identical right down to the embossed part number on the impeller itself.
I always recommend keeping at least one fully new replacement impeller on board at all times and a minimum of two gaskets as sometimes you'll need to check the impeller but it may not need to be replaced.