Thanks for the replies.

I have an image of the rear cassette and there are only 9 speeds. Maybe the bike is older than he is representing.

Could I use any 9 speed Shimano cassette? in the 11-25 range? (or 11-28)?

This is for a trainer, so all I need is a reasonable drive train and shifting (and fit). The 23 tires and wheels, or the twitchy handling, are not at all important.