Paddle tire or chains to increase traction. Drop the front forks down in the triple clamp. Many riders cut the forks shorter. A selection of sprockets to gear differently for various hills. Weight saving measures like removing the front fender, smaller gas tank, chopped seat, shaving metal parts, removing the cooling system.
Some riders use air cooled head.
High performance engine modifications. Stock a selection of jets, it's a long season. Add this to your list on building a good hillclimber. On big bikes you should put a trestle frame under the orignal frame to support the weight of the bike when you extend the swing arm, because the weight can snap the swing arm when jumping. It only add to the weight of the bike.