Frutos is correct - either speliing is legit. Gai prefers to spell it with one 'S' (e.g. his Twitter) However his registration papers have it with two 'S''s - hence why the club uses two.
Wasn't there something about the Israeli langauge not having such thing as a 'double s', and thus the Israeli spelling is Asulin anyway, but either's acceptable?
Or do I just need more sleep......?