4 is a crystal oscillator. 5 looks like a transistor, not a resistor. 
8 is most decidedly a microprocessor.
You're not going to be able to cooperate with the existing micro, so you may as well plan on removing that and replacing it with the Arduino. Before you do that, you need to analyze the signals going around and determine what voltage level is "safe". If the temp sensor and transistor both have readable part numbers, look up datasheets and find out how to power them. Trace the circuit and find out what connects to what.
This isn't trivial, and you'll probably ruin at least one while you figure it out. If you're OK with that, dig in and next time "don't do that", whatever it is you did to break the first one. 