Millers make a treatment...cant say how effective it is but that's what it's made for
http://www.millersoils.co.uk/automotive/classic-fuel-additives.asp
best bet is to strip it down, check the bore, check the piston to bore clearance, check the rings end gap clearance, replace the small end bearing. You will no doubt find the specs in the manual
Theres loads on ebay is yours different...whats TID14-78?
http://www.ebay.co.uk/itm/YAMAHA-XJ600-CDI-1990-/251142286714?pt=UK_Motorcycle_Parts&hash=item3a793f2d7a