You can't just tune a Johnson radio.
Parts must be replaced.
It is a bit of work, but well worth it.
I repair Johnson radios / repeaters for a living.
You can get a service manual from E F Johnson direct.
Look them up on the net. They can also tell you the needed areas to
change you the caps to put it in the ham band.
Good Luck and let me know how it goes.
Daniel KC4PLC