I don't think 120mm is necessary since Harimau is not designed to go head2head with MBT in the first place (CMIIW). Harimau will act more as an FSV so 105mm is enough (resulting in a lighter weight, more ammo capacity, and perhaps cheaper price per hull)
what if we upgrade the gun with 120, add more armor and perhaps upgrade gun stabilizer, engine, gearbox and suspension perhaps it can head to head with MBT