What operating Z?
If 50 ohms it should be easy. Just use higher Ui core material for the current transformer.
I would think type 77 or 73 or maybe even higher for 137 KHz.
I used type 61 (Ui=125) in my HF watt meter.
Thanks for the info... The Z is going to be 50 ohms & the frequency will be 150-200 khz. This is an industrial application and will not be transmitted into the "ether" The xformer sounds like a good place to build around on a breadboard rather than modifying an existing unit for H.F.