A standard type K thermocouple will work well above the temperature range you are talking about.
At 900F the output will only be around 20 millivolts, so you might want to use an operational amplifier to expand this to about 4 volts. Alternatively, you could use the ARef input and analogReference() function to scale the Arduino input to 25 or 30 millivolts full scale. Not sure offhand if one way would be better than the other. 10 bits of A/D would give you +/- about 2 degrees resolution, not counting the limits of error of type K.
A bare-bead thermocouple made of fine wire will give the quickest response time. Might be hard to seal at high pressures.
Since you are working in a combustion environment (oxidizing? reducing?), this can affect your choice of materials .... type N might be preferable for a reactive environment. Sheathing can improve resistance greatly but at the expense of response time.
www.omega.com has extensive reference materials on temperature measurement which may be useful.
Another thought: what is the thermal coefficient of resistance of your ignition coil material ?
If it is high enough, you might be able to measure temperature directly by measuring voltage drop at constant current like a standard 4-wire resistance measurement (or current at constant voltage if that is more convenient).