Beyond the basic "WOW, they're really different" the Impulse engines will practically need to be scratch-rebuild. I THINK I see two other things and would appreciate opinions.
1) IS the primary hull on the ERTL model as thick as the studio model? Assuming the 9' deck hight, 18' at 1:537 scale would be just under a half inch, which is exactly the thickness of the saucher hull edge. Now, we all know that that thichness would HAVE to be thicker then just the 18' hight of two decks. A 25' thickness would work out to .55" on the model. As I look at them, it seems that my model will need about a 1/16" spacer to thicken the deck-hight.
2) IS the Dorsal on the ERTL model as thick as the movie model? I think it was Griffworks that first mentioned it to me and in these pics, the ERTL dorsal doesn't look as thick. It looks like it could use being 1mm-2mm wider too.