i think a mod that should be legal in pre 85 is fitment of a disc front as long as it is within the realms of the period , my view comes from a safety point , running a bike with drum against disc bikes (esp. the faster classes)can be hazadous , unless u want to just follow them around , i guess at club level it doesn,t matter too much , the caliper on the 490 L above seems to be yamaha ,s 84 xt 600 , comments anybody ?