Conspiracy theories are not plausible in the eyes of media and ordinary people. It is better to tell them that the old temperature record is bad. Data is missing, inconsistent and created for another purpose.
Especially metadata, data of the data is missing. We do not know the history of weather stations accurately enough to reconstruct the information based on the raw data only. A temperature reading is a sum (or actually a complex function) of the climate signal and a number of other factors such as exact time of the reading, UHI and changes in the local environment. Without knowing these accurately we can’t calculate the climate signal.
This is why sceptics follow the satellite data that we have from 1979.