What kind is it (OTF, TTF, dfont, Type 1 suitcase & printer pair)? It it's the Type 1 pair, you don't need to do anything but copy them over. If it's a dfont, use dfontifier. If it's one of the others, get Transtype or use this free-but-fiddly alternative.
Note that OpenType fonts in particular can have extended character sets, automatic ligatures and alternates, dynamic metrics, and other cool stuff that will be lost when converting to an older format. Such is life.
.AFM? That would be Postscript in Windows-friendly format, that doesn't normally play well with OS X.
You can try ATM Light if ATM isn't already installed on the OS 9 machine for some reason. Or, grab the PostScript font (there should be more than one file, the PFB & PFM are the more important parts) and convert to a Mac-friendly format as before.
Oh yes, if it's just one font involved, if might be simpler just to buy the proper Mac Type 1 font (and if it's in PostScript, it's almost definitely a commercial one that you would need to license).