Brick chimney with no smoke chamber: is there some type of metal insert that would work instead of rebuilding inside of chimney?
Yes chimney liners are available depending on the local codes. Stainless steel and aluminum flex are common. You may need to contact the logset manufacturer to find out what is approved. If the mortar is in good shape you will not get better draft with a liner. Sometimes because it's not cool enough outside or your chimney is too short you don't get good draft until the chimney warms up. That's when a liner comes in handy. If sized properly it will warm up fast and draw within minutes. Talk to someone about doing the work for you as chimneys are not a do it yourself kind of job. You are dealing with carbon monoxide.