Your thermostat at 143* is proper for a Marine Carbureted engine.  It is possible your thermostat stuck and caused your overheat.  Never hurts to replace it if you want a reliable engine.

If you are new to the boat a new impeller is good insurance for the next two years.

Being an 80's PCM Chevy you should find a physically big thermostat under there. The part number is RP026003 and it carries a big price tag too. Let's just say 100 plus bucks.

These days it a 170 degree that they sell you, the 143 isn't available.

There's a kit that replaces the thermostat housing so that you can use the automotive style thermostat that's smaller, a lot cheaper and easier to find. It uses a PCM R026002 thermostat. The kit is RK025026. Not so easy to find the kit.

Cross your fingers and hope that the original housing was replaced in the past with the kit but that looks like the old housing that takes the big thermostat. 

I'd google both thermostat numbers to compare pictures to what you pull out, just to be sure you know what you have. Post pictures  to show what you have.

The raw water pump is a Sherwood G55-2 and the impeller is Sherwood part number 10615K available at lots of places.

I'd at least get a look at the thermostat, test it in boiling water and take the pump apart to inspect/replace the impeller if you don't know when either was last changed, along with the hose inspections etc. mentioned earlier

 

You have many great tips to follow above.   

If you have a new impeller, you can see how it is supposed to look.  Gathering the pieces you can tell when you could put it back together like a puzzle.  

Almost all of the impeller pieces were at the thermostat housing and at the inlet to the exhaust manifolds in his boat but some chunks can stick inside the hoses adding to your fun.

Since then, his wife keeps track of how old the impeller is and REMINDS him frequently what happens when he does not change it on time.   

Never dry start the engine, the Raw Water Pump is lubricated by the intake water.  Dry starts kill the RWP impeller super-fast.   Look into adding a TimmyT for running the engine on the driveway.  Makes the water hook up really easy.

(I admit, before leaving for the lake I will turn my key just long enough to see if the starter is turning with full battery strength or if I have a problem.  In my boat sometimes this quick battery check is enough for the engine to try and start.  I switch off quickly and hope all is OK.  Yes, I know I should not do it this way, but I hate finding a dead battery on the boat launch.)

Main pump....if you mean the circulating pump, nobody really checks that. If chunks got in there, they then make their way into the engine cooling passages, then you'd have to tear the engine apart and look in all the cooling passages

Engine inspection.....mentioned above, you'd be tearing it apart.

Look at the 90 degree inlets to both exhaust manifolds. Unscrew them and make sure there's nothing lodged in the fittings or in the manifolds.

And the suction side of the pump can also have pieces there, unlikely as it might seem.

So kinda like fanofccfan said, slap in the new impeller after a few checks for pieces and give it a shot

QUOTE=KENO]

Mpost wrote: 75 Tique wrote: Per Ken's request/suggestion, and per Ken's efforts in finding a relatively easy way to post photos, here is a step by step process. 1.  Have photos you want to post in your phone gallery or on your computer. 2.  Open website imgbb.com (no need to sign in, sign up, or register unless you want to) 3. click or tap on "select images to upload" 4. Go to your gallery or computer and select photo(s) (Leave autodelete option on "dont delete") 5. click or tap "Upload" 6.  In the "Embed codes" box, scroll to and select  "BBCode full linked" 7. in the box at the bottom of the page where the photo URL shows up,  tap or click "copy" 8. Open your CCF reply and tap and paste or click and paste. 9. Voila, its there. (Note: it wont show up until you view preview or post it) I suppose 8 steps sounds like a lot, but its mostly quick easy point and clicks (or taps)  Goes quickly and easily.  Give it a try. Added this here so it is sticky or pinned

I see it has the big, old, expensive thermostat.

Did you remove the elbows at the front of the manifolds to get a look at the elbows and the entrance to the manifolds

And  make sure the suction and discharge hoses are on the right pump connections, otherwise it won't pump any water and you'll destroy your new impeller.

You'll most likely be just fine.

I would pull the exhaust cooling hoses like Ken Mentioned and search for more pieces, you can look into the exhaust cooling inlet with the hose off and blow water back through the hoses. towards the pump while they are off.   If water does not want to flow freely you might run a long wire through to help purge your pieces of impeller.  You are apart now, just a little more time and you will know it is clean where you inspected.  You should be back up and running soon.  If your new impeller is correct and fits order a spare now and keep it in the boat.   I have not needed my spare yet but have shared it with other boaters a couple times while out on a lake somewhere.