Its a weird shifter on these things as i recall, there is no shifter drum with the usual detents. The gear leaver turns a ring and that affects the shift forks on a long pair of cross shafts. Made sense to someone? Looking under the bike my bet would be something on the round raised area in what looks like a sump but isn't.
That's the shift ring and the neutral detent will like as not be around there, wire looks about the right size. I'm scratching my head back to a yds i messed with with my dad when i was 15.
Hope it helps.