Not everyone thinks imitation is the sincerest form of flattery. Take Neil Diamond. When producers of the new David Spade flick Lost & Found asked the pop idol for permission to use his song ''Brother Love's Traveling Salvation Show,'' his handlers wouldn't agree until they saw a taped preview of the scene in which Spade lip-synchs and croons the tune. ''I did it exactly like I did it in the movie, but I sang the whole song,'' says Spade, who admits to being a serious fan. ''And we waited and waited, and they finally called and said, 'All right.''' Now, says Spade, ''I'd love to sing it in a concert with him.''