Just read on ABTW. They share most of the architecture. The 8900 has modifications for better anti-magnetism and METAS certification.
Yes, the idea behind the architecture is the same. 8900 is a slightly taller movement (5.50 mm vs 5.30 mm for 8500), some 8900 parts have different size but that's nothing I would care in case of a rep movement. There are the cosmetic difference in a placement of the writing and the colour of 8900 balance wheel is more grey than black.
Since revision G the 8500 has the same anti-magnetism resistance as METAS standard, however 8500 movements never got the official METAS cert.
As V6F nailed 8500 decoration I am sure they can cope with a nice 8900 too.