Is it a Mamba IR3. Not a Dynasty version though.
It still has the cobra insignia coin on the side - don't lose that!
If the board is still good I would just get a new battery. If both are bad, get a 9vlt board/battery setup.
If you just need that one oring, honestly I would just take it to a hardware store and find the next closest size for .39 cents.
They are rare, but not super rare. I would peg the value around 230-360.
It will be rare enough to make people at the field go