My unit is bigger than yours (snigger), so a new hydraulic damped replacement should fit straight on to my A frame I'd assume. It's a same the A frame on your trailer was built around the older spring coupling as even with the use of an adapter plate you'd have problems as you said, basically having to re-size all the components in the brake rod system to accommodate the linkage on the hitch being further back.
Out of interest have you towed with a hydraulic damped trailer, and if so is there any noticeable difference in the way it tows compared to the spring damped one?
I failed at the first hurdle with my trailer, couldn't even figure out how to connect the brakes up - they used bare cables between the hubs previously to some kind of pivot, and contrary to everything I read online my drums engage when you push the arm in the hub as opposed to when you pull it which makes no sense at all

I really need to sort it out
