You need at least 1 lvl 100 to fight the pumpkin king. He is one of the endgame bosses you can fight every 2 weeks, theres a guide to them here. https://pokemonrevolution.net/forum/viewtopic.php?f=89&t=9593
So are you able to come back to kanto to do things such as digging in mt moon or rock tunnel once you go to johto, or do you have to wait until you beat the 8th gym for that too?