impeller help please

[haulyard]

serial number 3656641 mercury 40 single cylinder 4 hp motor,was not pumping water ,tear down of lower unit i find impeller was toast,Now how do i figure out which impeller to buy,is it sierra 18-3039 ??

And how do you get old impeller off?? i have the housing apart but i dare go no furter till i here from you guys...

THANKS