"MD project"?
If you need more strength/stiffness but increasing the diameter and mass a bit would be OK, you could consider wrapping 3/4 oz glass cloth around a paper 13mm tube, using a good quality epoxy resin. Just I can't tell from your posts what the specific need is for a fiberglass tube.
Worst-case, 13mm drill rod could be used as a mandrel for making an all fiberglass tube. After a good coating with spray on mold release before layup, the ID of the resulting tube should work for holding a 13mm engine.