for this age of bike could be a number of things, low oil pressure causing lifter tap, piston slap, timing chain slap... just to nake a few that my old 80's bikes have before I restore them.... go with some 50wt oil if you're not up for tearing into the motor and see where you stand after that 
by the way how many miles are on this lil cc bike? done a compression check & leak down yet? that will def get you some good readings
by the way how many miles are on this lil cc bike? done a compression check & leak down yet? that will def get you some good readings