well, these numbers are used because some arabic constants cannot be written in latin alphabets... the numbers are picked so that they resemble the shape of the corresponding arabic alphabet
yes you can replace them with letters. the pronunciation won't be accurate though.
2 : a أ
7 : h ح
7' : kh خ
9 : s ص
9' : d ض
6 : t ط
6' : th or z depending on the dialect ظ
3 : a ع
3' : gh غ
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