It can be done. Check out Ben Hecks homemade Atari portables.
My suggestions:
-Stick with 68000 and use optimised software
-Get 1 MB extra ram
-Use a Compact flash harddrive
-Use a small TV/monitor then you do not need scandoubler
-KB and mouse converters are available
Problem areas:
-The case, if you want a thin case, get ready for lots of problems fitting this in an old laptop case
-If you decide to make a laptop case yourself get ready for lots of work
-Find out what kind of batteries a stock A600 can run on for at least 3 hours and see how many cells you need. Battery technology have come a long way since 1991. Also you can use a mini-ITX atx pico PSU, you feed the PSU with 12 v and it takes care of splitting 12 to 5v, gnd, 12v and so on.
Lots of work and probably even more work if you want to end up with a good looking solution. But it is definitely doable!
