If this occurs only under hard acceleration it could be down to worn selector components, which are outside the main crankcases and accessible with the engine in - so that would be my first thing to check. All of that mechanism has to removed anyway if you end up having to strip the whole thing. Worst case would be that the dogs on the gears themselves have got rounded over and spit themselves out of contact under acceleration. If that proves to be the case, then unfortunately it is an engine out, complete strip and rebuild, with all the costs of gaskets, seals etc. even if everything else, other than a couple of gears is good. You don't need many special tools, just the rotor removal tool and a special socket for the clutch centre nut, but these are precision built units and care is needed.
Hope this helps
Ian
