Welcome to the forum.
1994 was the last year of Rangers with the EEC-IV computer which used OBD1
There is only one O2 sensor on 4cyl OBD1, it is near exhaust manifold
EEC-V computers with OBD2 used a second O2 after Cat converter to make sure it was working.
So bad muffler won't effect O2 in 1994 unless if was blocking the exhaust from exiting system.
This video shows how to use CEL(check engine light) to get codes: www.youtube.com/watch?v=X07hu0kAuzE
Your engine is a 2.3l SOHC Lima, also called Pinto engine, 2.0l/2.3l/2.5l Limas all used the same block.
These are good reliable engines, almost bullet proof.
1994 was also the last year of a separate ICM(ignition control module), it is located on the front of the intake, and could be troublesome.
Newer computers had ICM built in.
On the 2.3l Lima only the exhaust side spark plugs fire when starter motor is active, intake side spark plugs will start firing when engine RPMs are above 400.
Once engine is running all 8 spark plugs fire and both spark plugs in each cylinder fire at the same time, there is no alternating.
And both fire on compression and exhaust stroke, Waste Spark system.
In the late 1980's Ford change all vehicles over to an oil pressure Switch, and added a resistor to the oil pressure gauge.
Switch closes at 6psi and Grounds the gauge, so oil pressure should be between 1/4 to 1/2, and stay there, regardless of RPMs, any changes on the gauge would be voltage in the system going up and down with RPM.
1994 can be changed back to "real" gauge, read here: Ford Oil Pressure Gauge Fix
The temp sender for the dash gauge is located on the drivers side head toward the back, an odd spot because it won't have highest temp, which would be at thermostat housing, like where temp sender is on most engines.
So yes temp gauge would be just at or above 1/4, on other Ranger engines just below 1/2 would be "normal" temp.
Thermostat should be a 190-195degF not 180degF, all fords now use 190-195degF thermostats, longer engine life, better MPG and cleaner oil at those temps.
In colder climates some run a 205-210degF thermostat in the Limas, they run too cool when outside temps drop, heater barely works.
They also add air blocking panel in front of rad to keep engine bay warmer.
On the back of the instrument panel is the Anti-Slosh module for the fuel gauge.
These do have problems, it prevents gauge from swinging up and down when cornering, most vehicles have similar circuits, Ford's just seems to have more issues that others.
Google: ford ranger anti-slosh module
There are a few video and articles about this.
With instrument cluster out you can also test wire to fuel gauge sender in the gas tank.
Yellow wire with white stripe runs from cluster all the way to the top of the fuel tank.
In 1994 Ford uses 16-158 Ohm sender, 16ohms Empty, 158ohms Full
With OHM Meter hooked up to yellow wire and a ground, rock the truck, OHM meter should show ohms changing as gas sloshes around and float goes up and down.
If OHMs are higher than 158 then wire may be broken or sender in gas tank is
If ohms are at(around) 16 and just stay there then float has a hole in it and has sunk to the bottom of tank
If ohms are below 10 then wire is shorted to Ground or sender is bad