Thank you for updating this to 2.0!
The mod works fine by itself, but with Space-Age active, the prototype step aborts on:
Error while running setup for entity prototype "express-underground-belt" (underground-belt): next_upgrade target (turbo-underground-belt) must have the same fast_replaceable_group (transport-belt != underground-belt)

SA adds a 'turbo' tier, and fiddles with the express tier's next_upgrade which is probably the source of the issue.

YMMV, but I threw together a tiny fix in a test mod that seemed to work for me:
data-final-fixes.lua

if mods["space-age"] and mods["belt-legacy"] then
    data.raw["underground-belt"]["turbo-underground-belt"].fast_replaceable_group = "underground-belt"
    data.raw["splitter"]["turbo-splitter"].fast_replaceable_group = "splitter"
end

Hi, thanks for posting! Sorry for late response but the factory must grow... Currently I do a vanilla playthrough before I play Space Age, so i can't test things.
Does it also work when you put your code in the data-updates.lua file? Or does it have to be in final-fixes?

It does work in data-updates.lua. (I tend to default to data-final-fixes.lua when I wanna test random things.)
I also have the mod setup with "? space-age" as a dependency, which may or may not be helping due to load order.

Update is released, pls let me know if something is borked.

Failed to load mods: __belt-legacy__/data-updates.lua:38: attempt to perform arithmetic on field 'space' (a nil value)
stack traceback:
    __belt-legacy__/data-updates.lua:38: in main chunk

LUA doesn't like - in field names. You kinda have to do mods["space-age"] for space-age and not mods.space-age.
PS: I have tested the update but with that change to mods["space-age"] and it is working for me.

oops, should work now :D