I'd recommend starting by carefully reading this post by Peter Norvig. (I had to something similar and I found it extremely useful.)
The following function, in particular has the ideas that you now need to make your spell checker more sophisticated: splitting, deleting, transposing, and inserting the irregular words to 'correct' them.
def edits1(word):
splits = [(word[:i], word[i:]) for i in range(len(word) + 1)]
deletes = [a + b[1:] for a, b in splits if b]
transposes = [a + b[1] + b[0] + b[2:] for a, b in splits if len(b)>1]
replaces = [a + c + b[1:] for a, b in splits for c in alphabet if b]
inserts = [a + c + b for a, b in splits for c in alphabet]
return set(deletes + transposes + replaces + inserts)
Note: The above is one snippet from Norvig's spelling corrector
And the good news is that you can incrementally add to and keep improving your spell-checker.
Hope that helps.