mrmac: finding a suitable donor laptop may be next to impossible. The motherboards in laptops are often very odd shapes even the diminutive A9 motherboard is wider than the A4 motherboard from 15 years ago. Then add in the needs for long availability, small batches and open specification and you'll be lucky to find an interested company willing to provide it.
If it were an easy task to make a laptop I'm sure one of the following projects would have succeeded already. Acorn Stork, Acorn Artisan, IMS Peanut, Riscstation laptop.