If you have the surgery for a reconstruction you should be able to return to Aikido. I am not sure a brace would be permitted though in any competition in Aikido though. You are seeing the lack of rotational stability that the ACL helps provide. No brace in isolation will stabilise for that entirely. A specific ACL brace and the surgery will be useful for activities like skiiing. The repair itself should be enough with good and extensive rehab to return to Aikido.