Re: Rear cross bar is in bad shape.
I'd like to know or see more of the the trailer and whole set-up. Is this used in salt water? dunked?
My rear cross piece rusted out in the middle after 20 years; it even collapsed down with the boat on it but no harm done. It was a bolt-on piece so I just bought a new one. I mention this b/c maybe you (or a welding shop) can cut out and replace the whole cross piece.
Had another trailer (shared ownership) and the last 3' of the side frame rusted out and it bent down from 3' up the frame. We had it cut off and a new section welded on; got another couple years out of it. it's not unusual to see this with salt water trailers that don't get dunked all the way. The ones that do, don't last.
Whatever you do will be temporary--maybe up to a couple years. Keep an eye out for a used replacement, uncludnig one with a free boat on top of it.
What you do should also be decided based on the use of the trailer: long haul, bumpy road, heavy load, frequent use, busy traffic-all point toward a better fix or replacement. Consider the safety issue along these lines; our patched together trailer was a "dry slip" situation where the boat stayed at our own ramp on the trailer, and at most occassionally went a few miles on the country road to the shop. Not the same as the weekend warrior.