Here is a picture of their Lower tube, with an Aerotech 24mm motor tube from a 29mm adapter kit, placed inside of the Vostok tube. It looks like the instructions show a smaller motor tube with centering rings, to go inside this lower body tube, as seen above in picture "VOSTOK3" item number 24. I have not found this tube, nor the centering rings in the kit, however I have not opend the smaller parts bag yet. The engine hook is in the parts bag and is the right length for a standard D engine. It looks like they did not ship me the motor mount tube, which is no big deal as I will fit it to the D engine as shown. With the pictured setup you can adjust their tube to fit our 24mm motors. I also included a closer picture of one of the "Outer" body tubes. The motor tube looks to have deep spirals to fill, the "Outer" tube spirals look to be very small, and hope to not show much.
Hope this helps!
Happy Flying!