If there's an original needle in the machine keep it, it's the best referance point for later. There might be a needle system in production that work with your machine, it has turned out so for Singer 12, 319 and others. The original needle can be carefully sharpened on a fine honing stone to even out the worn tip, but since it's needed for later measurments I guess it's better not to do it more than once. Keep the original needle and draw a red line just below the needle clamp. From needle tip to the red line is as far a regular 130 needle should be inserted in the clamp.