How I do to modify the contents of a ISO file?

You can use your Windows 98 Boot Disk (or any other bootable disk image for that matter) with ImgBurn (which you already have).

Choose Build Mode, add your files and then make it bootable, detailed instructions in this tutorial:

ImgBurn Build Mode - An Overview

You can use MagicISO trial version. It's has a 300 MB ISO size limit for the trial version. But it is still worth to try I think. It is still bootable after editing, I've tested it.

1. Mount (using Virtual Clone Drive for example) the .iso files you want and copy all their contents to a new folder.
   OR
   Use a program like 7-Zip to extract the .iso contents to a new folder.

2. Add/remove/modify anything you want in the new folder.

3. Use the free program ImgBurn to "Write files/folders to disc".
   

   • Select the the new folder and/or any other files you want.
     
   • Make the compilation bootable. Go to "advanced" tabs in ImgBurn and then to "Bootable Disc". Check "Make Image Bootable". Detailed instructions for that is here.

4. Burn to a disc.