plugs, cap, condensor, rotor, coil... (if you don't have the electronic ign. retrofit.)
plug wires... not a bad idea to change them every couple of years, its cheap insurance. 15-20.00 (I'll get slammed by people telling me that I shouldn't do this, but hey, it works for me) I go to my local tractor supply house and get some for a farmall, or a fordson tractor, makes no difference, the IH farmalls and Fordsons are 4 cyl gas 12v engines. Cost, less than 20.00 .
I'm not cheap, I'm practical.
most, if not all of these items are available from your local auto supply house, no need to pay for "marine" versions, they come in the same box!
for example, get an "internally ballasted" 12v coil (I'll run to the boat and get the number for you) and rip out the coiled ballast wire or the ceramic ballast thats on the wall. replace that with some 12gauge primary wire and have a happy life for less than 20.00
the impeller... it depends... I've seen 4 different pumps on a-4's, god only knows what your has.