Integer@mod() is the way to go here, to answer your question about multiple return values, SkookumScript has a mechanism called return arguments that you can read about in the Primer in the docs.
Blueprints and UE4 use references for return values (and SkookumScript can use this too).
MathLib.fmod() method would be called like this:
!remainder: 0.0 // Initial value to modify in call
!result: MathLib.fmod(1234 100 remainder)
println("remainder: " remainder " result: " result)
and it would print out:
remainder: 34.0 result: 12