can't answer your question with any certainty, but i'm curious about what you'll be doing there. you in the army or something?
somewhat educated guess follows:
as for peds in japan, i can only surmise that any old school peds have been used up and probably recycled. any old peds are prolly hard to come by and expensive there might have been imports back in the '50's, but probably not after that. you'll probably find alot of hondas and especially stuff that you've never heard of in the states.
other thing is that it's probably REALLY expensive for what it is. everything is expensive there. peds, scooters, and any other small vehicles are prolly also in HIGH demand because cars are expensive and too big to drive around easily. speaking of expense, if you need a license to drive peds in japan it might be upwards of $2000 to get it. if you're not living there for a long time, try getting an international license to save you much time and money! gas is expensive too, like $5 per liter or something.
check into the price on peds and availability there, i could be wrong about the whole thing. if i'm right, though, you should consider lining up a nice light ped for yourself here in the states and shipping it by boat to japan. you MIGHT be able to save yourself some dough that way. then sell the ped there when you leave. maybe make some cash... just make sure your ped is legal there.
finally, do you know japanese yet? if not, try the pimsleur learn to speak system. i'm using it right now and i really like it. i've taken college classes and used other systems, but pimsleur is best so far. pimsleur.com or check out ebay ( look up user bigredgarage).
i think japan is pretty neat. i'd like to go there and stay for a few months some time, but i don't think i'd want to live there long term. crowded. soshite moo takai desu yo.
let us know how you get on next year.