Actually, it's the outside diameter of the boosters that's nearly 13mm. There's really no way to fit a mini motor in the boosters, I'm afraid.
And, yes, I've done part of this PMC myself. Not the Energia-Buran, actually, but rather the Hercules kit from Start/STC. I have a couple of their Energia kits as well, and these all use the same parts for the main rocket body and the boosters.
Estes 18/24 centering rings fit right into the main body of the rocket, if you peel off the outer green layer of paper. I CA'd one into each end, then used liquid plastic model cement to glue the two halves of the model's body together.
I have some more notes in this thread , by the way. No pictures up yet. The fires here in San Diego are pretty distracting...I'll get them up once things calm down.