if you get the 68 bore cub, you have to either have your stock head cut for the bigger bore, or if you run a after market head you have to buy big bore domes. To get 90-100 hp, you are looking at bigger carbs, drag pipes, reeds. timing, lock up, override etc.... Oh and a bunch of time testing and tuning, cause with out that you will be wasting all that power.
What do you plan in doing with the bike??