I would recommend buying a new combination. Even if you still can get the same make there is no guarantee the trolley will fit the trailer. Wayfarer trailer building is still largely done by hand and a trailer and trolley are usually build as a pair to ensure a good fit.
If you really want a trolley only, your best option is to supply your trailer to the builder and ask him to fit the new trolley to your trailer, or have him modify the trailer to fit the new trolley. In either case I don’t think it matters much which builder you decide on.
But what is the state of your springs and dampeners? Your hubs and bearings? Tires?
If all those are in need of some serious TLC a new combi may be a better choice in the long run.