OB-1 back up battery

[FENATIC at aol.com]

Gettin set to do some fiddling with my bbq'd OB-1, and I pulled the corroded
back up battery out of its holder. Its an old mercury 4.2V T-136 (i think).
First electronics shop i hit couldnt come up with an exact replacement, and
suggested a 4.5v that was a different size.  They told me to check another
outfit to try and find one more physically similar.  Is the voltage difference
a problem?  Is there any permanent damage likely from this batterys demise? It
appears the corrosion only ate up the battery holder connection on one end. Is
there a source for the correct battery?  Thanks.