I'd suggest you contact Brownell's for either a replacement or a Belt Mountain base pin. Maybe a generic will work for you.
As to a frozen base pin, probably it has been peened by impact against the base pin latch. Remove the base pin latch, cock the hammer and try to drive it out with a wooden dowel from the rear of the gun. Sometimes if you can push the base pin in further so that it protrudes out the back or the frame slightly, just pull the trigger and let the hammer drive it out. After removal, use a fine whetstone to smooth down the pin. The area requiring stoning is just behind the circular groove.
If this does not work, remove the hammer and drive the base pin out with a wooden dowel and a mallet. It'll give.
Before trying all of the foregoing of course, make certain the gun is empthy.