Go back and talk to the man in the center of the upper room, on that platform ledge thing, and give him the Machine Part. He'll restore the power to Kanto and give you (another copy of) TM57 (Charge Beam). That's cool, because one just isn't enough or something? Oh well, thanks anyway.
You'll know you did the right thing if you get TM57 from that guy in the middle of the Power Plant. If you didn't and he sounds sad, you're probably forgetting the Machine Part, which can be found by searching (press A) the red-and-white inner tubes in the northwest corner of the Cerulean Gym after beating Misty. Make sure you pick it up from there!
Now that you've restored some power to the land, you can go back to the Kanto Radio Tower and talk to one of the officials in there to receive the Expansion Card, which is necessary to pick up the Kanto Radio Stations. You must do that, so go on and fly over to Lavender City and pick it up.
You can also take the Magnet Train in the northwest corner of Saffron City back to Goldenrod City in Johto. That's probably the quickest way for you to get back to Johto at this point and it's much more convenient than having to run south of the Pokemon League down to New Bark Town or so.
Anyway, the place you really want to go back to after getting the Expansion Card is Vermilion City. You're about to dig it up in the Diglett's Cave!