The service manual you got is for the domestic models. I have quite a few parts book for the Japanese models, but not yours. I can check with Japan for it. I can supply parts, but without part #s it is pretty hard to get the right parts.
It is a Japanese model from the 80's. It was shipped to Viet Nam to be fixed good enough to sell, painted to look like new, & the hour meter turned back to 0. Some of the viet "rebuilds" are good tractors. Some are real nightmares.
I have some of the L1 parts books, but not the L1-20. I can probably get you one from Japan, if you buy the tractor. Some parts will probably be available in the US, but some will probably have to come from Japan. Email if you are interested.
I would recommend getting the correct parts book for your tractor. Not the closest domestic models book. There are some differences. I hate paying a restocking charge, because I ordered the wrong part. We have most of the older grey tractor & mini excavator manuals & some newer ones. I can get...
I can supply a new reverse idler gear, I'm sure for a lot less than fixing the old one. Welding teeth back on & machining usually doesn't work real good. On parts that aren't available or expensive, we grind off the bad gear, press & weld on a sleeve, & use a gear cutting machine to make new teeth.