I think adapting the curriculum to meet their needs could only be seen as “Reasonable Adjustments”. If they are not even working within the KS1 levels there is no hope of them even understanding the KS3 curriculum.
Do you have a special school nearby? If so why do you not see if they would give you a copy of their curriculum?
Also I would teach these pupils in a Primary School format – i.e. one room, one teacher, TA. It would even be worth appointing a Primary Teacher for this role. I have worked in Aylsham High School in Norfolk recently and they have a superb “Nurture” setup like this for their low attaining KS3 pupils – give them a ring to discuss (they like talking about what they do there).
Good Luck
John