Machining the bottom of a Conshelf to use the removable volcano orifice is not an option. There is not enough space or material. The replaceable volcano orifice is about 0.28 tall. You would need that extra space. It is not happening...
You can try polishing the original orifice. A lot depends how bad is the scratch/ nick. For very light scratches, I have used a pencil eraser. If you don't have access to one of Herman's tools, you can use a wooden dowel with a little piece of very fine sand paper stuck to the end. Just rotate it very lightly and do a little at a time. Check it often and go slow.
Used Conshelf on eBay are always for sale and most of them are going to be OK. Many of them have never been service, which is a very good thing. That means there is much lower chance that they were damaged by a human...
I have purchased many used Conshelf and very rarely do I see a bad one.
Buceador con escafandra autónoma clásica.