A new battery at the shop opposite the post office will cost you approx 50 pesos. I don't know about the strap because I guess it will depend on type. The wife of the deceased owner has very tiny hands and hasn't ever butchered anything she has done for me. If you're not feeling too confident in Spanish yet, one of the daughters is learning English - I don't know how good it is because we speak in Spanish, but it would probably be enough to overcome any hurdle.