Both the Montesa Cota 25cc, 50cc and the Bultaco 50cc Chispa, did not use their own in-house engine design for these small trials bikes. The engines were built by MotoTrans, a small Spanish company that produced engines under Ducati license and is no longer in business.
As Steve mentioned; Engine parts, original bodywork and the correct bits and pieces for this bike are unfortunately, extremely hard to come by.