Though of limited application, I have sometimes installed specialized aircraft fasteners, as opposed to regular ones, to deter theft. Stopping theft is probably impossible, but making your property the least attractive as a target has merit.
When I purchased my all terrain vehicle many years ago, a thoughtful friend explained to me how easily three phillips screws could be removed, and its ignition bypassed, so it could be pull started, and driven away. The screws in those locations are no longer phillips, and will require a specialized aviation screwdriver to remove.
I'm sure that the authorities would be understanding and allow a fastener change, to a different suitable aviation quality fastener, if doing so would prevent theft. If engine mount nuts were to be replaced with the appropriate version of these:
When installed, the hex snaps off. Special tools are required to remove them - but that's the point! Yes, it would make engine removal a little difficult, but how often do you do that anyway?
