I had a 1975 70 hp Evinrude in about the '95 ish time frame. It had a tstat but no OT sensor/alarm. The root cause of my problem was a stuck shut stat and nothing to tell me the engine was overheating till one day at WOT it locked up.
First off, with low hours in answer to the spun hub I doubt that would be your problem, unless you damaged the prop with a serious underwater strike. But as Searider said, perform a prop test to verify. Simple as pulling the prop, making a scribe or permanent marker line from the inner diameter of the prop housing (where it slips over the prop shaft, to the outer diameter (where the hub and the prop outer casting meet. Take it out and run it hard. Then pull the prop off and check the line. If still straight prop hub is ok. If not, it's slipping.
A slipping prop is identified otherwise by an increase in engine rpms for no increase, or a decrease in speed just like revving your car in N sort of thing. With a rubber hub, when you have a hard underwater strike with the prop, it will break loose and rpms will shoot up and speed will drop to nil. Returning the throttle to idle and shifting to N usually allows the hub to reseat allowing you to be on your way again like nothing happened. Too many of those and it won't reseat properly. A plastic hub (currently popular separately sold hub for a given prop housing) will either hold for the strike or rip apart inside and you can't reset it, have to replace it.
On temp, the tstat is the controlling element for operation under 2500 rpm (serv. manual. number). The pellet should be stamped 143 for 143F opening temp. That's about the temperature of a domestic hot water heater in your home set to the Normal temp range for a benchmark. You can test it on the stove in your kitchen in a pan of actively heating water with a candy thermometer. Recommended way is to manually crack the seal and insert a string or thread from which you suspend the stat in the water as it is heating. When it reaches opening temp it will fall from the string. After that the next 10+ degrees it continues to open till fully open.
The popoff is a pressure sensitive valve and allows increased water flow through the engine at speeds above 2500 rpm. Testing that would be more complicated.
You can remove the cowl from the engine and initially on your day's outing, make a good 10 minute, more or less WOT run. Put your hand on the top rear of the engine block and mentally record the temp. If operating properly, you should be able to keep your hand where you put it.
Continue with your outing and periodically make the same test. If you find that your having to take your hand off the engine because it's getting too hot, you have cooling problems. Increased ambient temperature will reduce the performance of your engine due to less dense air ingested, but not as much as you indicated.
Oil mix is 1 part oil to 50 parts TC-W3 (Two Cycle Watercraft, 3rd iteration, 1 pint to 6 gallons gas). Yours would run ok on W or W2 but you can't buy it today and no need to. I doubt you have an internal oiling system with that yearmodel so that is where you set your pre-mix. Wallmart for one, has it in the sporting goods section, in Pennzoil or Merc/Quicksilver brands. Pennzoil works just fine for half the price and is semi-synthethic which makes for low smoke.
Once you work through this, come back and we'll take you from there.
