Recurse on the first argument for addition and multiplication, and on the exponent for exponentiation:
add :: Natural -> Natural -> Natural
add Zero     n = n
add (Succ m) n = Succ (add m n)

mul :: Natural -> Natural -> Natural
mul Zero     n = Zero
mul (Succ m) n = add n (mul m n)

power :: Natural -> Natural -> Natural
power m Zero     = Succ Zero
power m (Succ n) = mul m (power m n)
The defining arithmetic identities are
(1)
Each recursive call consumes one constructor. The base of exponentiation makes power Zero Zero equal to one, the empty-product convention. Applying convert and using structural induction verifies all three operations.

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