In all the years that I've been around these bikes, and that's a long time, I've never heard of anybody who has retrofitted a Goldwing shock to them. I would be very surprised if that is an option but if so, would sure like to hear about it. It might open up a lot of possibilities for used shocks. That being said, if your Hagon is the heavy duty model, I was told that you could send it back to them and they will reconfigure it for your needs. I have no idea what they charge for that.
Now I'm also surprised to hear about the low volume on the stereo. I will be the first to say that the 4" speakers in these bikes greatly limit the sound but volume has not generally been an issue. Most of the time the problem is that the speakers start breaking up, can't handle, what volume the system is capable of. You see the volume is very low. How low? Are you sure you have it set on speakers and not headsets? At any rate, after trying several brands of speakers and even a small amp, I finally accepted the fact that I was never going to get the desired sound out of the 4" speakers. We now just use Edsets headsets without the speakers and just plug earbuds into the jack on the headset module. That works great for us.