That piece the seat front bottoms screw into is simply a large, bent sheet of aluminum. You have several options including trying larger diameter sheet metal screws to taking the piece out and tig welding in aluminum nuts on the inside. If you have a good surface still, you can try a #10 or 1/4 NFT tap and then use stainless machine screws with loc-tite. Be sure to use fine threads to get more bite in the aluminum.
Bottom line on those is not to over-tighten as the aluminum will strip out.
I took mine out (99 TR21) a few years ago, cleaned out the fuel tank, changed all the fuel lines, etc. Cheap insurance.