Do you have a shop manual for the 1300? I don't know enough about them to offer any advice but I know that you have to have a main ground that runs from your battery. You can trace that with the wire. Then you might look to see if you have any bolts that are screwed into the frame with a black wire coming off of it. If so then this would be a grounding point. If there is still paint on the frame then that's what needs to be sanded and greased. I would guess that if you look from your spark plug wires then they should terminate into coils. The front and rear coils are bolted onto the frame under the tank - guessing from the 1800. The place where the coil is bolted is the corrosion point. Just take the coil off, sand the frame, sand all the negative wires attached to that bolt, grease it up and that should take care of the ground fix. If I had pictures of the 1300 then I could probably point you to where the grounding points are. I'm sure someone on this board with a 1300 has a shop manual or has done a ground fix that could help with the ground locations.