This is a cross reference sheet that came with my tractor. The hand written notes are mine.
If I remember correctly the NAPA thermostat was a few thousands of an inch too large on the outer diameter to fit in the housing. I gently filed the edge all the way around until it fit properly. The one problem I had is the old bolts that go through the housing were in really bad condition. I think two of the four twisted off and I had to extract the left over bits. Never any fun. I put in all new ones with lots of anti-seize compound on them.
It certainly wouldn't hurt to go through the head bolt torque and valve adjustment procedures.
Some of these tractors were assembled with fully painted starters which acted as insulation. The only ground they were getting was through the mounting bolts and the little bit of metal the lock washers scraped back.
It never hurts to take every electrical connection apart and clean all the surfaces until shiny metal is exposed. Starting at both battery terminals and working outward. You would be surprised how much resistance can build up on a dirty connection.
If you have the time and desire, when you get to the starter, while you have the wires off, remove the starter. Do the same shiny metal stuff at both mounting bolts and under the bosses where it contacts the bell housing. The more surface contact you have the more amps can flow through it. Also, take the starter apart and have a look inside. Clean all the contacts in the solenoid and at the commutator. A little white lithium grease on the sliding/spinning bits will help a bunch. Don't forget the bushings at each end. You will be surprised at how much more power your starter will have. All just simple maintenance stuff, just a little time consuming.
Another thing to consider is internal cable corrosion. I have seen cables that look completely fine on the outside be so corroded on the inside that they would pass only a small amount of current. For an example: The positive battery cable gets moisture inside and corrodes. You test it with a multi-meter and get battery voltage at the far end. This is fine when there is no current flowing through it, but as soon as you put a load on the cable, it becomes a big resistor and all of a sudden you are only seeing 8 or 9 volts. The tendency is to think "oh, oh, the battery is bad". That happens but if your pretty sure it is not that case, use a set of jumpers to bypass the vehicle cables.