it sorta looks like a rt100, but from my experience anything yamaha always has yamaha on the clutch cover.. and since it has a plastic tank and suppose to have shrouds, one would think its a +86 model bike.
also looks like it has a different muffler (and header pipe is differently shaped), rear shocks and kick start on it. oh and rt100 from memory are 5 speeds.
bike bandit lists the rt100 as starting in 1990
lets start with looking at the base of the barrel somewhere, it will have he engine size embossed or stamped in it..
also there should be some sort of faded yamaha printed on the seat cover (if its the original one still).
this one shares the same front guard and some other features and it sorta looks like you can just see the yamaha on the clutch cover
1993 Yamaha Rt100 - Boostcruising
higher quality pictures would definitely help pin point the model.
1 thing to consider tho is if it is a rt100 there not really worth anything, so decide if your doing it up to keep or for re sale.