Pull the broken ceramic fuse out and see if there are any part numbers on the ends or underside. You’ve got nothing to lose. If it’s indeed connected inline with the AC input, 15A sounds about right since the specs for you unit say 1300W/130V (10A) though it also says to use a 20A circuit.
If still in doubt, install a fast blow 15A. The worst that will happen is false blowing. That said, slow blows are often used where more than brief over current is expected like with an inductive load inrush current. I’d guess fast blow is most likely in yours.
