I'll second BigO's technique. Those DC computer fans can be even smaller. I used a smaller one and attached it right to the base structure of a K-Line/Marx type on the far side, not as easily seen. Works like a charm.
I've also taken a very small DC motor, and put a piece of plastic (placed slightly off center) on to the armature of the motor. I attached it to the underside of the upper base of the tower using self-adhesive tape. The vibration makes the beacon top spin at a slower pace than does the fan.
But like BigO illustrated, the distance of the fan from the tower, will undoubtedly affect the speed of the rotation.
By the way Gandydancer, good to still see you around on the forum. Your 027 reefer cars are still an inspiration to me and I have one your posts on that bookmarked. That was a great project idea.
I've got some set aside on the project list, but am finishing up an 027 Alco FA repainted to the experimental Century Green NYC paint scheme. I like that paint scheme, however temporary it was on the NYC. When K-Line did the scale E-8, I was hoping they'd do the Alco FA in that scheme. Ah, sometimes you just gotta do it yourself!
And an 027-ized Lionel U-Boat shell, cut and chopped down to fit a K-Line MP-15 frame. The first Conrail one came out so well, this is my second one of this project... only in NS this time. I've always wished Lionel had made a modern scaled down 027 equivalent to the 1950's Alco FA ... they didn't. SO I DID! 
