I guess the question is how original did you want to keep the radio ?
For example if you replace the ballast with a 50L6 and add a 330 ohm resistor across the heater you would effectively turn the tube into a 300 ma tube instead of a 150 ma tube.
The resistor would be 7.5 watts and I am sure a 10 watt one would do very well.
If you had the space, two 25L6’s in series would also work without a resistor but you need another tube socket.
Just food for though from my slightly off brain !
John G. VO1 CAT