You'll be fine. All of the 'you need to be a jockey to race a 125' talk comes from people looking for excuses...
Last time I raced the Nationals, four of the top five placed riders in the pre-78 125 class were at least 85kgs. Most of them were on those shitbox Suzukis too, which handle well but have a motor that is well short of the best in the class (the 125D).
At HEAVEN events, most blokes in the class are full size: Noel, Snowy, Scotty, Oldstuff, crossedup2, Donny, me, etc etc. When I race in the all-in class, the blokes that beat me are better riders - their weigh is irrelevant.