The first Doctor only having one heart is very much fanon - it might have been an assumption of the Virgin books, but it's just a theory, and you can take the onscreen evidence either way. For instance, when Ian says 'His heart's beating,', that could just be that he only checked for one heart, on the left, because it didn't occur to him to check that the Doctor might have another one on the right.
Yea, regardless what onscreen evidence there is, because it hadn't been thought of yet, it makes no sense not to retcon The First Doctor as having 2 hearts. Time Lords have two hearts, we later find out The Doctor is a Timelord, therefore, The First Doctor must have 2 hearts
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?