Recently replaced the cam and associated parts with the motor still in the car, on a black (shows scratches well) Backdraft.
This worked well and not a single mark when done, with lots of opportunities to really make a mess.
Go to Hobby Lobby or similar fabric store, get the proper length of heavy duty clear cling vinyl 48" wide. Clean the finder well and then put the clear vinyl down. It's thick and will cling like it's glued to the fender. That alone will prevent any scratches since nothing moves agains the pain from that point. Just cover the vinyl with whatever your want, even the fender protectors mentioned and you can drop about anything but the whole motor on the fenders and you will be fine. When done, peal the vinyl off, roll it up and wait for the next time.....and there will be a next time.
By the way, I used the same cling vinyl under the edge of my soft top and it prevents any scratches or rubs completely, peels up easily and goes back on when needed.
Good luck