Lets break this down into two pieces.
The seat belt light: All this needs is a switch in the seat belt buckle. The switch in the buckle should be normally closed and when the seat belt is inserted the switch goes open. The seat belt bulb then is wired to a key on source, then to the switch and the other side of the switch is grounded. That simple. Assuming that the switch is already there, a bulb and wiring is all that is needed.
The low fuel light: This one is more complicated. The fuel level sensor sends a voltage to the dash that represents the level in the tank. What you need is a comparator. The comparator takes in the signal from sensor and a reference and then outputs a high or low. Just as an example; lets suppose 0v=empty, 5v=full and 0.5v is low. The comparator would get a reference signal of 0.5v. The comparaotr would get wired in such a way that when the sensor signal drops bellow 0.5v the output of the comparator goes high which can illuminate a light. That is the simplest. There can be lots of improvements from there. Like some delay and hysteresis. This would keep the light on even when the tank sloshes. This kind of circuit could be built up on perf board with some simple to moderate electronics and soldering skills. If you want to get fancy a circuit board could be designed and all the parts installed. This solution would be $50 to $100 bucks.
The problem with using junk yard parts is that this kind of solution is built in and not conducive to just excising the low fuel light circuitry only. The second problem is that the circuit would be adjusted to the specific car. You would have to reverse engineer and modify the adjustment to match your car.