There is no reason that any CD drive you buy would not work with this system. There is no such thing as "requiring" Windows 98 2nd edition. Maybe the software that comes with it to burn a CD (if it is a CD-RW) but nothing is required to just use a CD drive, the drivers are all built into Windows 98, even the old version you have.

There is no Mhz requirement for CD drives, they will run 1x or 2x if that is necessary, or 40x or higher if your system can support it for the function you are using.