Can you post a photo of your thermistor wiring splice?
From the motherboard, unplug the thermistor cable and measure the resistance of the (cold) thermistor. To know exactly what resistance it should be, you’d have to know what model of thermistor is being used in the new hotend assembly.
When you say you smelled burning, did it come from the hotend, the cabling, or the motherboard?
Unplug the hotend heater, but leave the thermistor plugged in. Turn on the printer. Read what it shows for hotend temperature.
Measure the resistance of the hotend heater. Again, you’d have to look up what the cold resistance of that particular hotend heater is supposed to be.
Just keep in mind that the controller has no idea what the either of the heaters is actually doing. It can only measure the thermistors and make the assumption that the thermistor value is a function of the heater temperature. A defective thermistor or thermistor wiring can give false results that are not indicative of the actual heater temperature(s). That having been said, you smelled something burning, so there’s obviously too much heat somewhere.
How quickly did it reach that temperature? I get the impression (maybe falsely) that it was very quick?
Also check that the target temperatures for the hotend (PLA and ABS settings) are sane. Just in case the temperatures spiked like that because and abnormally high value was entered and the printer is in fact behaving normally in trying to reach it.