hope you didnt cut the grass and fire a rock into your compressor unit or hit the lines with lawnmower or weedeater cause it sounds like you got a refer leak
you should be able to replace whole system for with a mor efficent 3 ton unit for 3000.00 for the works. You already have ductwork, cu lines wires etc so it should be even cheaper.
The pine t and g toe nailed direct to the joists is sub floor. They did it that way back then as plywood was scarce and expensive. Your roof and exterior walls will likely be the same t and g material. It made for a very strong well built house.