Have you turned the engine over, gently?
I don't think they can get near, but a pal with an old Matchless 250 found after one winter that he could not fully turn the engine over, but he can rock it, just not as far as TDC. He suspected the little blighters had been storing nuts and seeds in the combustion chamber, reached via the exhaust pipe and valve (there was no air filter damage - a non standard item).
Unfortunately, he's now retired and I have not heard whether the head is off the engine yet...
Cheers,
