1. Place pork in large resealable plastic bag. Add 3/4 cup of the ponzu sauce. Seal and refrigerate 1 hour.
2. Meanwhile, whisk together remaining 1/4 cup ponzu sauce, lemon peel, lemon juice, honey, ginger and red pepper flakes. Set aside.
3. Coat grill rack with nonstick cooking spray. Preheat grill to medium-high (350°F to 400°F). Remove pork from bag; discard marinade. Grill pork 10 minutes per side, brushing occasionally with ponzu sauce mixture, until instant-read thermometer registers 155°F for medium. Remove pork and let stand 10 minutes. Slice pork and arrange on platter. Top with fresh cilantro, if desired.