In my opinion, crypto is the easy part. I think the only problem I had when I worked on my own Accounts&Sync re-implementation (a crude hack, I admit) was some MAC-related issue with BrowserID, where different (old) Firefox versions had generated different assertions. Don't really remember what it was.
The real problem is data model is significantly different. It's possible, but a lot of boring work, designing the appropriate transformations.
The real problem is data model is significantly different. It's possible, but a lot of boring work, designing the appropriate transformations.