I'm too lazy to find the URL to point you to my suggestion on ML, about Goldelico ("Openmoko") GTA04, slightly modified to match N900 so much that we could use original screen (LCD+digitizer), original housing and keymat, and almost original fremantle.
Honestly why re-re-re-invent the wheel? There are free open documented hw designs out there already, the problem is manpower to do such development, particularly on OS level (and obviously apps). Alien-dalvik (android compatibility layer)? Where? Who?
Multitouch? HAHA, suuuure. Since all maemo devices and their GUI design have multitouch support since ~6 years¡ You for sure know that a (limited) MT-functionality can get implemented on current N900 as well - just go ahead and do it! Or find somebody else willing to do it.
Honestly I'd already appreciate when somebody would start resp resume on manufacturing 100% "original" N900 again. Changing to a larger RAM, or faster CPU is really the least problem (if you still can source the chips).
Don't get fooled by delusions about the amount of effort needed to develop and manufacture a complete smartphone incl 100% feature complete and reliable production grade software.

/j