Spokes come in lots of different lengths and I couldn't imagine you having too many difficulties in Japan sourcing any size you needed.
Saying that, I've not been to Japan and am unaware of how rural you'll be so if your worried, take a few spares.
From memory, I've three of each size inside my handlebars.
I think you'll be fine.
Examine your spokes around your cassette as its surprising how much damage a chain that has skipped over the largest cog of your cassette can do without you spotting the damage.
As a one owner bike you'll know if this has happened.
If there is no damage to your spokes, I'd get them tension checked at your local bike shop (ask others in your area for a recommendation) and you should be good to go.
36 is a good number of spokes in a wheel and 110kg total is not much of a load.
I think you'll be fine having had the wheels tension checked and good to go (with the caveat of your spokes having no visible damage)