there nothing wrong with drum brakes, my DT50 has drum brakes front and rear and its not too bad for stopping.
gie the brakes a good clean out, fresh set of shoes and it will do you a good service, theres no point in "pimping up" a bike which wasnt designed to be pimping in the first place.
keep the SR125, once you have thrown it down the road a few times and got to grips with biking you can get something a little more elaborate