i've done a 24", 1940ish Colson, a 1942 Colson Imperial, a 1930's Colson Flyer, and a late 30's/early 40's Hawthorne Twin Bar Zep.
and i've got another early 40's Colson Olympic i'm ready to build, but i'm waiting till summer ends, 'cause it's an awesome beach cruiser.
the bikes i build start off as a basic platform, covered in rust, and would cost more to restore than what i could get back out of them, so i don't feel bad about slapping a chinese 2 stroke on them. i also don't do any modifications to the frames, so as long as they don't crack, there's no damage to them, so if anyone ever wanted to restore them, they could.
i would draw the line at motorizing an original, complete, near-perfect classic, though. there's enough basket cases around so i wouldn't have too. i mean, unless i found the perfect, vintage motor that would compliment the bike. then, anything goes.