Dismantle the thing and see what needs replacing? Pawl-springs, pawls, maybe new bearings while you're at it. For sure that can be done with a freewheel block- can't imagine this can be much more fiddly, but then some people don't even want to go inside a freewheel block.. :? If you already have a replacement wheel, and the freehub on the old one is not working, what's to lose?