I don't own one or race Enduros but I know of a few resources on them.
here is some magazine tests I stumbled on when I was looking to increase performance on a DT175,some of the tips and tricks is very useful for any off road bike in these.
http://yamaha-it.co.uk/text/lit.htmwhen it comes to parts and literature Ebay is your friend here.
http://www.ebay.com/bhp/1980-yamaha-it-250here is a link to Yamaha so you can look up part numbers,this helps greatly,especially on parts searches on Ebay.just use the drop down menus on the LH plane to look up parts.
https://www.yamahapartsandaccessories.c ... s=Off-RoadI hope this helps!