I gather the microphone/earphone connector is not compatible?
If you want to use your old helmet in civilian aircraft. Could you cut the plug off the end of the cable and buy a new plug for that aircraft? Then get an electrician to solder the new plug onto the cable of your helmet? The cables should be the same, it should just be the plug that is different.
http://www.microair.com.au/admin/upl...andsockets.pdf
headset connectors | Mendelssohn Pilot Supplies