Surgeons usually recommend healing for 6 weeks after abdominoplasty before resuming full activity. That is how long it usually takes the healing scar to reach 75% of it's full strength. It is still more vulnerable than normal tissue so extreme activities should be avoided for longer. Gradually resuming full activity is the best way to avoid injury.
If you were my patient, I would say after 6 weeks you may ride your motorcycle. I generally allow my patients to return to full activity, including sit ups and weight lifting at about 6 weeks time. You should still ask your surgeon what he or she recommends. Every surgeon has a different routine. Good luck.