Don't get me wrong, I'd love to see you try it, but I don't think it will work. Because of the transfer gear module on the 3-6-9 on the A7750, it is slightly thicker than a 7750 or 7753. The crown and pusher holes have been adjusted for that, and I don't believe they would align properly with a 7753 in that case. From what I have heard, the 6-9-12 case is different as to accommodate the unmodified A7750, but I am not sure if the 7750 and 7753 are the same depth.
To make this work I think at the very least you would need the following:
Serie Speciale (6-9-12) case
2004 Navitimer (3-6-9) caseback
Genuine dial (needs to be ground down about 1mm as its too big)
Lello datewheel (provided it will fit in a 7753)
Of course you will need to AR the crystal as well
If all these pieces fit together you should have a decent franken-Navi, but the individual digits on the date will still be pointing in the wrong direction, and unless you can find an old school case, the slide rule markings will not line up properly.
Hope this helps, its been a project that I contemplated for years, but the risk cost was just too great to take. As you already have the most expensive component, the ETA 7753, it may be something that you could try. I'd contact The Zigmeister for clarification as to whether a 7753 and unmodified 7750 have the same dimensions before you even begin.
Cheers,
A