fatass is apparently a "stripped down fasttracker 2 clone"... luckily FT2 is one of the biggest and most popular trackers ever so it's easy to find clones of it, i would try:
- fasttracker 2 (for MS DOS)
- milkytracker - a beefed up fasttracker 2 clone for windows/linux/mac
- nitrotracker? - a stripped down fasttracker 2 for NDS
- protracker 3 - fasttracker 2's granddad. for amiga
^those ones all use sampled instruments so you can do what you want soundwise
- reality adlib tracker (an FT2-feeling tracker for MS DOS fm synthesis) - the instrument editor is a bit confusing (same with all dos fm trackers) at first but you get your head round it quick enough
someone else can probably recommend trackers to run natively on atari or spectrum
the c64 trackers seem to have their own cliches and ideals and as far as my experience goes, none of them feel much like FT2 (that's not to say you won't like them!), in fact they are more similar to lsdj mostly as the creator of lsdj is a c64 scener. but cybertracker seems popular for its simple PC-like interface
just spitballing here because i haven't used fatass so i don't know how it differs from lsdj but i guess the patterns-as-a-whole vs patterns broken into chains and phrases will be the key interface difference given that the GB basically determines the input scheme... patterns-as-a-whole is by far the most common approach in PC trackers, far less common on C64.
good luck!