To make the brine: In a large bowl, whisk together water, salt, and sugar until solids are dissolved. Submerge pork loin in brine.
Place in refrigerator and brine for 2 hours.
Place peppercorns and fennel seed in a cast iron skillet over medium-high heat; toast spices until fragrant, about 2 minutes.
Transfer to a spice grinder and process until coarsely ground.
Transfer spice mixture to small bowl and mix in sage, rosemary, garlic, thyme, crushed red pepper, and lemon zest. Set aside.
Lay pork belly, skin side down, on a large cutting board. Score flesh with a sharp knife at an angle about every inch. Repeat in opposite direction to create a diamond pattern. Season pork belly liberally with salt.
Sprinkle rub evenly across pork belly, using hands to pat rub into meat and cut crevices.
Remove pork loin from brine; pat dry with paper towels.
Place in center of pork belly.
Roll pork belly around pork loin so it fully encases loin. Tie roll close with butcher twine about every inch.
Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and arrange the coals on either side of the charcoal grate and place a foil pan between the two piles of coals. Cover grill and allow to preheat for 5 minutes. Run spit of the rotisserie through middle of pork roll and secure ends with rotisserie forks.
Place on the rotisserie, cover, and cook at medium heat until skin has darkened and crisped and pork registers 155ºF when an instant read thermometer is inserted into the thickest part of the meat, about 3 hours, replenishing coals to maintain temperature as needed.
