That's a difficult one. If you are sure that their mother is "brainwashing" them, I'd consider calling in the child protective services or talking to your lawyer. As a father you have certain rights, and, more importantly, your children have certain rights concerning you that their mother must not be allowed to violate. But I'm no expert in this respect and the focus of your question lies elsewhere.
My answer would depend on the circumstances of your reunion. How much time will you have with them? Will you be alone with them, especially without their mother? When and how often and for how long will you see them again after that first meeting?
The more time you have, the more slowly you can take it. You could even consider calling in a psychologist to accompany you, especially if you think you'll have to deal with lies and "brainwashing" from their mother.
Being recognizable in the way you have described (by bringing things they might remember and care about) is certainly a good strategy. Given the way that memory works in autism, you might want to consider wearing something that they might be familiar with. (I don't mean that autists don't remember faces, but that some of them have a good memory for seemingly insignificant detail that others forget, as well as a usually high need for continuity, consistency, and constancy.)
As a warning, you should definitely prepared for the possibility that meeting you again after such a long time will be quite overwhelming for your children. Depending on their attachment pattern, they might not immediatly be open to you, independent of "brainwashing" and recognization. It can well be that they'll react with aversion or seem not to care about you, so it is good if you are ready for this and give them the time and space they need to get used to you being there after your absence.
All in all, the best thing you can probably do is:
- give them time and
- keep your own fears under control.
Be relaxed and open. This is not (only) a return to something previous, but the beginning of something new. And building something new will take time.
And finally, if their mother is present, try to keep whatever is between you two out of your reunion with your children. Your ex might want to use the moment to make accusations or remind you of your shortcomings. Ignore anything unproductive and negative and remind yourself of why you are there.