Hi Steve, I've been doodling a little with scale and numbers.
1. I don't know the scale of your model, but I would guess it's 1/48, or 1/4"=1'-0".
2. One source called it 1/35, but it's not listed on the MiM website.
3. In answer to the question about the shuttle engine nozzles at 1/144, if accurate they should be 5/8" diameter at the mouth of the bell.
4. Your little shuttle kit has nozzles about 1/2", correct?
5. FWIW, that scale drawing shows the nozzles to be 48" diameter.
6. Based on 1/48, you are looking for something 1" diameter. On 1/35, around 1 3/8".
7. So the shuttle nozzles you have are too small, and the ones you found online are as well.
8. Correct me, but aren't these under water? If so, you can be rough with them unless you are setting up a submerged view.
9. One other comment. That drawing shows the "re-entry" engine nozzles attached to... nothing. I didn't take intergalactic engine design in school, but the guess is that the engine would occupy some space, and then there's fuel.
Just my two bits.