Yeah, if its any concellation: been there, done that too (but it was already some 15 years ago
).
I replaced the entire bottom of the front chassis with the same new piece thats available for it and what you have inthe above picture. Nice new 2,0 mm thick piece of metal. Even cut and rewelded the center reaction arm mountings to weld it in as one piece.
While I was there, I also welded in a boxed frame for more stiffness of the front chassis, which is s/th the 1303 can really use, when you go circuit racing with it later
.
So, just hang in there! Someday it WILL be finished and you'll own one of only a few really thorrowly restored and improved 1303's.