I have same problem but i tried out this

void main() {
  print(fixNumber(8123456789, amountOfZeroes: 4));    // 1000056789
  print(fixNumber(9665789456789, amountOfZeroes: 5)); // 1000009456789
}

int fixNumber(int number, {required int amountOfZeroes}) {
  return int.parse('1' + '0' * amountOfZeroes + number.toString().substring(amountOfZeroes + 1));
}