Easy to do in Sigil if the headings use a unique 'class' (the only use underline + italic implies this is so)
Then you could just change the <p and \p> to a <h3 \h3> and have Sigil generate the NCX (and HTML TOC if desired)
I do have 3 clips saved to build searches with
Code:
(One|Two|Three|Four|Five|Six|Seven|Eight|Nine|Ten)
(Eleven|Twelve|Thirteen|Fourteen|Fifteen|Sixteen|Seventeen|Eighteen|Nineteen)
(Twenty|Thirty|Fourty|Fifty|Sixty|Seventy|Eighty|Ninety|Hundred)