I used the spelling one of my mates has (Alister) but I think that is one of the less common ways of spelling it so it's understandable. I haven't seen the question again so I've not been able to see if other spellings work.
Alexander works; it's English for Alasdair.
Handy to know, when doing Scottish genealogy! Much like knowing that Jane and Jean both translate to Sìne -- which explains why those names get used interchangeably for the same person throughout her life.