Sounds like it's the clutch splines, an often neglected area of maintenance. The splines need greasing, the frequency up for debate, but at least every two years or 10000ml is a starting point.
It may just be the clutch splines gone, in which case you are only up for a new driven plate. If it's the gearbox input splines, then I'm afraid it's a new shaft and a total gearbox strip (well, at least enough to get the shaft out, but while there a good time to overhaul it). Not cheap if you are getting someone else to do it, so a known good SH box may be the answer. You can also probably pick up a good SH input shaft.
You can only tell for sure what is gone by pulling the box from the engine, no biggie, but how are your wrenching skills? Keep us informed as there are some traps for unwary players if you haven't worked on Airheads much before.
Bill.....................;-)
