This is something we are working on. Five years ago, a car was not allowed into the Legend Certification unless it had the oem drive train. If we do let cars in, they will take a hit on the replacement engine as it wouldn't be fair to people who have cars with an original engine. This is why the oem cars have a high value. Camaros that survived the past 38 years or so with the oem drive train deserve to have a higher value.
There still should be a place for cars with restoration blocks. I don't think CE's will ever have a place in the concours arena unless there is dealer paperwork. It's getting hard to find anything today, but the cars are still out there. I just looked at a Z28 two weeks ago that was in storage for over 30 years sitting in a barn in Gettysburg, PA.