System 7.3 does not exist. Do you mean 7.1, or perhaps 7.5.3?
First of all, I assume that Disk First Aid is on another (startup) disk, not on the hard disk that you wish to repair.
If Disk First Aid cannot repair a disk, it is often necessary to use a third-party program instead (such as the commercial Norton Utilities).
If the hard disk does not contain important files, an alternative could be to just reformat it. Try Apple HD SC Setup 7.3.5, Drive Setup 1.7.3, or a third-party formatting utility (maybe something like Lido 7.56).
Norton Utilities v 4.0 would be the appropriate software to attempt repair. Maybe you can find it on eBay.
It was announced on 1998-09-14. For the first time, the tools are now run on native PowerPC Macintosh computers. It adds support of Mac OS Extended Format and HFS+ file system.
It supports PowerPC Mac (up to G3) using Mac OS 7.5 and above (up to 8.6).
Here are two links to possibilities.