Damaged brass seat is caliper???

Just screw a longer sheet metal screw into the tube seat, and put a plyer, or vice grip, under the head and then hammer it up, and out.
As far as finding a source for tube seats, good luck with that.
Not easy finding a source for them.
At one time, i needed one or two of them, for a pair of calipers that i was rebuilding, and couldn't find any.
A member on this forum had some in his personal stash of parts, and sent me two of them.