to the forum
I love the xs400 sohc, there are only general tips i can give.
just don't throw anything away as it invariably is the the part you need 6 months down the line, or someone wants to buy it from you!!
the camshaft runs in the head with no bearing surface, so always keep the oil topped up and use the best 20/50 you can get.
you can use RD stuff as the bikes chassis are similar and can also use some engine parts (rd400 D shift shaft is one)
the bikes are simple beasts get a parts list ( or the online one from motogrid/yamahatopdog/bikebandit) and there are manuals links around the forum (yamagod is the one who has these at the mo')
if you get stuck, search the forum first as we have discussed lots of xs400 problems over the year or so. If your still stuck, post in here and you'll get lots of info (good and bad )
drewps