Check the big suppliers listed in the links under Member services at the top of the index page here. You should find after market vent liners at Dixie, TOTW, Log Cabin, Cains, and other major suppliers, including Brownells. The Chamber's coned liner works better because the cone is parabolic, like the curve of a inside lens. It helps to focus both heat and pressure in the powder chamber from the gases that are trying to rush back out the vent hole at ignition, delaying that exit, increasing chamber pressure which in turn increases the chamber temperature to ignit the powder faster. A straight sided cone simply directs the venting gases out the small hole, and does not reflect gas, heat, or temperature back into the chamber.
No matter what liner you choose to replace what you have, you are going to have to find out what thread pitch is being used, and then acquire the proper sized drill and tap to prepare the barrel for the liner. There is then further work done with either file, or machines, to reduce the length of the cone on both the inside and outside of your barrel to make a correct fit.
As to the size of the vent, much has to do with the location of the vent in relation to the flashpan, and whether you are using FFFg or FFg powder in your barrel. I first opened my vent to 1/16" but found it still was misfiring on me-- the dreaded " flash in the pan". I then opened my vent up to 5/64" and that cured my hangfires and misfires with that particular lock. However, as already noted, if I decided to use FFFg powder in that gun, I would prbably start with a new vent and keep it at 1/16, unless I still got flashes in the pan. Only then would I open it up to 5/64" knowing that FFFg powder is small enough in size to permit granules of powder to pour out the vent into the pan when loading. I settle on FFg powder in that rifle long before I opened the vent to 5/64".
The important thing about all this is to make the decisions about your particular gun in the right order and at the right time. Find out what powder shoots the most consistently for you and your most accurate load. Then, if you are getting flashes in the pan, consider opening the vent up a little. Don't overdo this, and open the vent only to the next drill size. Try the gun out, and see if that cures the problem. It was only after shooting my gun over 6 months in three seasons that I decided that 1/16" was not going to cure the problems, and I bought a 5/64" drillbit, and open the vent the next step. My particular gun has the vent hole bisected by aline drawn across the top edge of the flashpan, a bit low for best performance. If I could do it over, I would have the vent drilled so that it was slightly above a line drawn over the top of the flashpan. Then, that 1/16" vent hole size might have done the trick for me.