If I was in the USA I would agree with the above advice but the cost of shipping from the UK is likely to exceed the value of the unit.
Adams Aviation are the UK agents and carry a good range of parts, the unit is not difficult to work on and as it sounds like a mic problem why not change that and see if this works, But first just check the continuity of the cables incase you have an intermittent conection.
The Adams website had the IPC for all the DC headsets last time I looked so you can find the part numbers yourself.