Give it to your dad for free.Apparently it has an aftermarket bar on it, it’s an oregonView attachment 873550 nothing comes up when I search for that, not sure if it’s obsolete or what
That chain is nearly new. Check other side of the chain. If it has a number will give guage/pitch. Drive sprocket/rim will also be marked with chain pitch
20" 3/8 bar will take 72 drivers and most likely 0.050 gauge. Chain looks to be 33 RM Stihl semi-chisel