Yeah just buy another one, they seem plentiful on ebay. Heres one for a BIN of $5, looks like you get the bolt and everything
http://www.ebay.com.au/itm/1977-Yam...rcycles_Parts_Accessories&hash=item563af9026b
tha's got the original head on as it is small, the updated ones are larger. BTW the bolt is the same for xj600 too (pre-diversion)
http://www.ebay.co.uk/itm/Yamaha-Oi...rcycles_Parts_Accessories&hash=item256e6d9f1d