Help Engine knock

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Ok first off I'm new here and also new to boating. However I am not new to engines. I recently purchased a 1988 rinker 186 captiva with a 4.3 mercruiser. The boat was previously owned by an older couple and seems to be very well taken care of. Now to the bad news last time out it seemed to be low on power and didn't want to idle like it was starving for fuel. So I changed the plugs and put some sea foam in it. When I pulled the plugs they had a small amount of oil on them which worried me but I chalked it up to pulling oil out of the valve covers since it was starving for fuel. I took it out today for a little while to see if it would run better with new plugs and after about a half an hour it started knocking so I babied it home to try and save what I could. Now I havent pulled anything apart to check it out yet but Im just looking for a little insight from people that have been there and done it already. The knock sounds like its coming from the head broke valve or spring maybe. Still has good oil pressure and I checked the oil first and it was still full as well. I have been looking into doing vortec heads and changing out the intake to be able to run a four barrel and seems now this would be a perfect time. Also if something is wrong on the bottom end I will probably bore and stroke it with a complete rebuild any insight on rebuilding it back to stock or go ahead bore it stroke it and change the heads to vortec with a four barrel sorry this is so long just trying to get the whole story out. Thanks in advance if its too much I will start multiple threads thanks again