Intuniv, Strattera, Concerta, and Vyvanse can be prescribed for ADHD in Japan. Ritalin is also available but not for ADHD, though travelers can bring a maximum of 2.16g of methylphenidate. Other stimulants such as Adderall are not allowed.
Most doctors would be willing to prescribe a different medication for the duration of your trip. It of course won't be ideal, but it would be a temporary hiccup that might be worth it considering you seem to be really excited for the trip.