...Replace with a new frame is out of question. Here is what I do, and still in the process of reinforcement my 1300Trillium frame. .. Pull frame off the shell. Flip it upside down. One hand with a chisel hammer, other hand with a manifying glass, starting from end to end, every inch to inspect the old welds, rusty/suspected areas and pay attention to those curves, those tubes at heavy traffic/load areas whether they have holes or bent. Then, complete welding those...half-way, unfinished welds, adding steel rails, L shape if necessary. After that, welding extra triangle steel pieces to those important corners(keep weights in mind, no excessive weights added if not neccessary). Welding is not always GOOD for a frame and sometimes bolting is in fact BETTER. Too much weldings on a frame could weaken it's original structure(learn that from 18wheeler manufacturer). After that, I woud flip it back to normal, inspect the top...then might add square tubes/ U shapes as new aluminum cross members in those heavy traffic/load areas. After all done, sanding all or sandblasting depend how rusty it is before
painting them. I also disconnect ground wires, sand those areas to the shiny bare, reconnect them and...grease them. One of my pictures..I really appreciated with all inputs/advices because I am actually one of a ....non-professional of any kind. Thanks