Knucklehead

Total novice here! New to the forum. Thanks in advance for any help! My grandfather build this pool in 1960. He had a great place and I'm trying to restore it. The pool is 20 x 45 and has been empty for the past 11 years. There were quite a few cracks in the plaster and some hollow spots in the plaster when we did the "tap test". I ground 3/8" grooves where the plaster was cracked about 1/2 - 3/4" deep and put silicone sealant in the bottome of the grooves. we then chipped out all of the hollow plaster areas. We then acid etched the pool and plastered all of the repaired areas. We refilled it and I replaced the pump. Got it all plumbed, replaced the sand in the Sand filter... and away we went. The rust flowed from the inlet pipes until it finally became clear. The pool held its level for a couple days before I got the circulatory system going. Then it started dropping. I'm pretty sure one of the metal pipes is rusted through. The pool was built so that it sits about 2-3' higher than the pump/filter area. For the skimmers (2) and vacuum ports the pipes go about 5-6' under concrete before they come out where the pump/filter are. So, I have a bunch of questions. I know I need to replace all of these pipes. The skimmer and vacuum ports are straight shots. Is there a good way to do this without busting concrete and digging? In either case, what is the best way to replace the metal pipes with PVC? Specifically, the fitting to the side of the pool? I imagine the skimmers would be no option but to bust the concrete and dig. Any thoughts or recommendations would be greatly appreciated! Thanks, Bill