You'll likely have to make your own. The headset is not wired for stereo, so all you'll get is mono unless you completely rewire it. You should be able to make a cable with a jack for a U174 on one end and a mini plug on the other. The speakers are wired to 2 & 4, the mike to 1 & 3. If you look at the plug, with the tip on the right and the cord on your left, the mike is the tip end and the second ring in. The speakers are the first ring and the base.
The GA speaker plug is mono, & you should be able to get an adapter from Radio Shack or similar to convert the 1/4" plug to a mini stereo plug. Buying an adapter to convert the helmet to fixed-wing plugs, then another adapter to the mini plug will be more expensive - expect to pay ~$US100 for the GA adapter, & maybe $5 for the mini adapter.
These things are pretty worthless for stereo, or anything needing high fidelity. They aren't the highest-quality sound reproducers you can find. Even very cheap stereo earphones will be many times better. I have in fact used cheap stereo earphones in headsets, but civilian avionics require impedance matching. Military aircraft use low-impedance avionics, so you shouldn't have any problems there. It just won't be very high-quality sound.