The LM35 is very easy to find. Any local store should have it.

The problem with non linear thermistors is the math.
I dont know how to program in a way that the result is always accurate from the "real life".

The LM35 is linear and accurate and is capable of temps up to 150º. You can easly program and get true results.
For applications as the one you want ( model engines ) , you will need a sensor that can go over 135º.

The only problem i see is to get the package of the LM35 fit inside the engine fins.
You could sandpaper off a bit of the package but you will lose accuracy.

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