If after the conversion your Cordovox Accordian complies to the standard GM midi tabel, you could use smf (soundfonts), for which you need a creative soundblaster card in your windows 98 pc.
This setup will probably run without many problems if you have stripped your windows pc from all other software.
The other option would be to use the roland SC88 softsynth in which case you need to virtually connect the softsynth. You can use midi yoke for that.
As a project I think it is possible, but not for live gigs
A windows 98 PC is not stable enough, not fast enough, lot's of latency etc....
If we look at todays softsynth's they won't run at all on w98 and you need at least a fast PC with a lot of ram memory.
I don't know the psr-220, but most Yamaha keyboards make use of the XM(yamaha) midi standard, which means you also have to have the possibility to change in XM midi table. (cheque your manual for that)
To make full use of the midi conversion, your accordian also needs to send sysx messages.
I think it is a hell of a project, but tempting
You might place your message also in the section Software arrangers of this BBS.
There are some very knowledged people there.