Note MP3 format is a compressed format, so to make the file small the file is compressed so some sound quality is lost.
But there are different quality levels of mp3 so if you do make them into mp3 format then choose the highest quality your can (320kbps for example).
When mp3 files are put on to CD they can be put on in one of two formats:
1) As mp3 files. These just go on the CD as "data" files so you can usually fit many hours of mp3 files onto a single CD.
But this CD cant normally be played in a standard CD player, though many CD players do allow you do play mp3 files..
2) As Audio files. You can burn a set of mp3s onto a CD as "Audio" files.
This is the same format as CDs sold in the shop, but you can only fit an hour of "Audio" files on to a CD. But you can play this type of CD in any standard CD player.
Note there are formats that are different from mp3 called "lossless" where no quality is lost, but the files are bigger.
FLAC is one lossless format (I think there are others), so if you do want to keep the quality of these cassette tapes consider copying them to your computer in a lossless format
https://xiph.org/flac/