What are the best sources for Hungarian paganism and what remnants of pre Christian religion(s) exist(ed) in the Christian period?
In English you mean? That is going to be difficult.
If by any chance you would be able to read Hungarian archeologist László Gyula's "A honfoglaló magyarok élete" (the life of Hungarians at the time of the land conquest) is a seriously comprehensive work of lifestyle, religion, customs, culture, agriculture just about everything. Note: the methodology is based on 1) cementery analysis 2) comparison with similar people like Kyrgisistan. This was done in the early 20th century. Is this methodology still acceptable today?