Unsure what output voltage they are they will be running no more than 4.5v (3 AA batteries). If you are looking at an after market motor you will have to find some one that can supply you one that is the same size, voltage etc that will fit in the rotor gear box!
the dye web site link below......they are £20, give dye a call at least you will know it is the correct one and will work correctly.