First, an admission: I have not done the experiments, and I haven't given any thought to possible conventional explanations. It seems clear that Leedskalnin was a construction genius, but I do think it rather likely that a conventional explanation is possible. The theory we have does describe nature extremely well, and it seems unlikely that such a discrepancy in the basic laws could be missed. What I can say something definite about is the role of magnetic monopoles in the conventional theory. It is true that Maxwell's equations could be rather easily modified to include magnetic charge. The equations actually become considerably more symmetric if magnetic charges are allowed. Also, as Dale mentioned, monopoles are a common feature of many field theories, and it is actually a challenge to understand why we don't see monopoles (this is related to the need for something like inflation). Monopoles also have the attractive feature of helping to understand why electric charge is quantized. So yes, detecting them would definitely be Nobel prize material, but it would hardly constitute an upset to the basic laws of physics.