Er, good to hear about the angles, I trust they work out! Perhaps in the interim you could humour me for a moment?:
Because of my background I have both a personal and professional interest in 'seeing what cannot be seen'. Geophysics forms a significant part of this and the problem you have falls, more or less, within a present branch of research and development that I've been pursuing for a while. This work is one reason why I come back to a magnetic survey methodology for locating the 'plane within your search area.
I've cast around and have been interested to find that there's not a lot of data on magnetic aeroarcheology - larger scale prospecting yes, but not so much the sort of thing I think would be required here. It may be that my search was all too brief, but from first principles (my username is an allusion to this!) I'm not especially surprised. Much of the successful work in the the archeomagnetic survey field has been carried out at ground level, and at relatively close range, after all the anomalies one is looking for are likely to be fairly insignificant and a high degree of spatial resolution is required.
Although the resolution issue may be dealt with to a degree, traversing the ground at any height will severely reduce the ability of sensor(s) to detect these anomalies that are usually underground - obviously the deeper underground the anomaly (object) is, or the higher above ground the sensor is the more difficult this will be. However in the case of most archeology the sort of disturbance that produces the anomalies generally sought is what I'd call secondary. There's no need to go into this in detail here but the mass of metal that you're looking for is a primary interest and should thus be much more evident in any survey conducted at a similar level, and the range of detection would extend to a somewhat higher level than usual in typical archeological applications.
Now, I have some feeling of the terrain and cover that you have in your area of interest, but obviously no personal experience - I wonder what the typical cover height of the foliage is? At what height agl could a small UAV traverse across the area without impediment but maintain a relatively constant distance from the ground?
Where I'm going with this is that my (fluxgate) magnetometers should detect a mass of metal the approx size of a P&W single wasp engine at between 15-20m distance. These are certainly not the best magnetometers available but they are what I have experience of and my view is that, configured as a gradiometer and carried at a distance beneath a UAV, they would probably 'see' such an object at a reduced height agl (let's say 10m for the moment).
From this, and while there are a number of factors that could affect the outcome, a properly conducted grid survey, at a reasonable height agl (less is obviously better!) could stand a fairly good chance of locating the motor and/or complete aircraft you're looking for if they're within the survey area. To be clear; this would be a series of physical measurements plotted and later analysed with appropriate methods - it's not something you'd use like a hand-held metal-detector.
This may not be a 'fit' for whatever you have planned, and it may well be that there are other reasons why it wouldn't work as I hypothesize, but I'm interested enough to want to think more about the problem, at least abstractly... It may also be that someone viewing this has significant experience in this area; any insight would be welcome!
FP.