I see that the SourceForge download page includes an installable version 5.7, but not a portable version. Is a portable version still available that works on Windows 98? Or can a portable version be made from the installable version 5.7? If so, how?
I am sorry that I don't know the answer to your question. I don't even know if there ever was a version 5.7 portable; I haven't found any clear references to it on the web. You could try the portable 5.8.8, to see if that runs on Windows 98.
If all you want to do is clear an old Windows 98 machine, you could try DBAN. Alternatively (and more messily), you could take the drive out of the Windows 98 machine, attach it (using a caddy or whatever) to a more modern machine, and use Eraser (installed or portable) from there.