would the magnetos provide some signal? Also the crank on a radial is going to be wrather short compared to an inline engine.. Is the magnetometer signal a function of geometry as well as mass?
If 20m range on a wasp crankshaft could be reliably demonstrated would it be technically possible ( not necessarily financially) to grid the area into 20 x 20 squares and "dip' the magnetometer as in dipping sonar from a helo?