Jazakallah khair.
Answer
It is permissible to make miswak while fasting. To use tooth paste in the
state of fasting is Makrooh (disliked). The minimum taste in the miswak is
inlike the strong taste in the paste. Furthermore the paste is a solid
substance and liquidifies more in the mouth thereby having the potential of
going down the throat.
And Allah Ta’ala knows best
Mufti E. Desai