I had a left over decomp that is oem quailty. I took the one that came with this one and used it on my 660. It fell apart when I was milling those few boards I did. I used the backup in this one. I tell ya removing the shroud on this puppy is a pita. You will want a top of the line valve to keep from removing it.
Are you on about the top cover when you mention shroud? How is it a pain in the ass? My 52mm on my OEM saw is plugged/no hole ie. OEM 52mm magnum 038. It does not need a decomp and it has no base gasket too. I would just plug the cylinder. I have both top cover and air filter farmertec parts. They both go on and off sweet as a nut.