You can re-encode the MP3 at a lower bitrate, which will reduce the file size. You'll get some quality loss though, especially since you're recompressing an already compressed MP3. If you have the original tune in an uncompressed format (.wav or whatever), converting that to MP3 will give you better quality than recompressing the existing MP3. Give 128 or 64 kbps a try when compressing, that should make it fairly small without making it sound awful.
You're not going to be able to convert it to a mod or xm file, well, not easily anyway.