RAD in Brisbane can do rebuilds of the OEM shock, and can also change the spring if you need a heavier one. This would be the cheapest option, but I have heard that the life of the rebuild might vary, but it may do what you want. I have a spare OEM shock you could borrow if you don't want to be off the road for too long (I think it is my original, but as I have lent it to a few people it may not be!). After that Hagon make replacement shocks that seem good value but not the latest or greatest.