Spy : I would have made the vision+attack effect the same as original and put the +1 card after, no matter if it's not base friendly
I have tried a card that gives the effect on Spy without the attack portion, and it works for $2 (being sometimes better and sometimes worse than Vagrant). The important thing is that the decision on whether to discard or keep a card loses room for guessing and assuming. Instead of "will the next card be better or worse than this Copper", you just choose the better revealed card to discard for opponents and the better to put in your hand for you.
Thief : Attack is perfect (a bit heavily worded), but I'd have added only +$1 not to make that card too exciting !
I only changed a single sentence in the middle, the rest is like the original. That early wording looks weird, right?
Either way, i figured the fact that Noble Brigand could also attack engines (clogging them) justified giving two coins instead of one on Thief. Sure, Thief can steal Coppers, Platinums and Kingdom Treasure, but this potential use case doesn't sum up to me as bigger than Noble Brigend always being able to attack on buy. As Noble Brigand is still not strong, and Thief can still be more easily avoided, i figured it was fine. It's not like the second coin is actually needed, though, so admittedly it's more than absolutely necessary.
Rebuild : Fine but too different from original !
I took this from Donald's
retrospective comment on Rebuild, where he explains which card he would have put into Dark Ages had he known how overpowered the original is. He mentioned it being well playtested and balanced. As i stated in the OP, i completely scrapped original Rebuild.
Feast : maybe "you may put it on top of your deck" would be better, for late-game Duchies... no ?
Good point. That's better
About your question, probably not. I have no acceptable way of replacing cards with mockups that don't feel worse than original cards. If i had, probably yes for Rebuild, Scout and Spy. Not sure about the others. It's not like i'm the only one playing.