Well usually i say start with the impeller in the drive, replace and then run the boat still overheating do the t-stat.
In your case i would yank out the t-stat leave it out for now close up the housing and run it on the water. No more overheat you found your problem.
Now say the t-stat is good, and the impeller is good then you have a problem down stream.
But first lets do some research and you can do this on the trailer. Put water to the drive, remove the long hose that goes to the t-stat housing, from the transom, just loosen the clamp on the long hose that connects it to the t-styat housing and hold the hose straight up. Start the boat, how high is the water shooting ?
Should shoot straight up about 12-18 " Do this for about 4-5 seconds and shut off the motor.
Good water to the t-stat housing ? No there is a clog before the t-stat housing understand.
Could be the impeller broke up in the drive and is blocking the hose at the transom then you will have to get the clamp at the transom loose and remove the long hose from the transom. You can do it, take your time and think, think, think before you do anything.
Ok you have the hose off, did alot of pieces of rubber fall out ? Thats your impeller. Bad thing, bad, bad, bad thing understand ?
Clear the hose ??? No rubber pieces, good thing. Understand ?
Ok no rubber pieces put back on the hose at the transom tight clamp. Only use a screwdriver and not a socket you can destroy things.
Take a garden hose and backflush the long hose to the transom. You want to see what comes out of the drive understand maybe backflush the hose with the drive in a bucket.
The backflush wash anything out ? Only you will know and maybe your dad cause i ain't there, look see what came out.
Ok say it's clear. No little rubber pieces.
The t-stat is good or new or not in there cause you yanked it out and left the houisng with no t-stat for now and there are no little pieces of rubber clogging the t-stat housing cause you looked and put your finger in there and checked.
Whats the oil look like ?
Go ahead run the boat on the water.
Does it still overheat?
If you find rubber pieces DO NOT RUN THE BOAT ! DO NOT RUN THE BOAT !
You have a impeller in the drive that broke apart, UNDERSTAND.
We can walk you thru replacing it. It's easy.
What year is the boat you left that vital piece of info out. Don't know ?
Look at the transom numbers the last 2 will tell you the year. If you post back please include year.
Understand grasshopper ?
please post back what you find.