If you want to replace the 6X5 (the easy way) with a solid state replacement here is a link to one that you just plug in.
http://www.tedweber.com/wx5
Trouble with those tube's is that they are known for heater to cathode shorts, and when they short it takes out the power transformer. Also Using diodes and resistors should work as well, but you might want to leave the filament in circuit for appearance. Not sure what value resistors are needed as I would assume it depends on the current draw of the radio.