Were you able to find a manual for the tractor in Japanese?
A google search for "Mitsubishi MT1401 service manual" gives the first result as a website that advertises manuals for this tractor. (They are not a site sponsor, so I don't think I'm supposed to link there directly. The manual is $98 plus $8 shipping. I suspect the MT160 is the US version of the MT1401, though I don't know that for sure. There are numerous copies of the MT160 manual on ebay.
What is wrong with your tractor?