Depends on why its not supplying pressure. The air is going someplace. It may just need the leather piston seal re-greased...
It can be used as is, just don't pull back so far.
It can be badly repaired with duct tape and spray paint. just to keep the top on, it doesn't effectively improve pumping.
You can come across replacements, both used and NOS, paint as needed.
Eric

PS I have a couple NOS leather seals...