1 -- clean remaining pins *carefully* so that you don't break any more pins off. I use Tarnex, slight rubbing then rinse.
2 -- Get yourself a 28 pin machine pin socket (machine pin sockets have exposed contact areas on top).
3 -- Insert remaining pins into new 28 pin socket.
4 -- solder the IC into the top side of the socket (not too much heat as the plastic frame will melt and pins might move)
5 -- for broken pin(s), add small wire from socket entry hole and solder directly to remaining pin stub.
Best wire to use for this is a small, solid wire such as a lead cut from a 1/4W resistor.
6 -- the 28 pin IC socket is now part of your 28 pin IC. Insert that into your existing IC socket.
To keep pins from melting loose and moving while soldering, I actually insert the machine pin socket into another socket to hold the pins in place.
If there are old legs stuck in existing IC socket, you may need to replace socket.