Well a 300 ohm resistor would tell the ECU that the trans is always warm enough, OK for testing but probably not a good idea long term.
It looks like a 2.2K thermistor would do the job as long as you can mount it so that it does measure the heat.
Not a real common one (not available at Jaycar) but this one is available from Mouser, and others are also available. This one will be roughly 2.7K at 20C and 0.275K at 80C.