As others have said, get the exhaust system checked for problems. Why was the engine replaced? Could oil contamination have 'strangled the CAT'?
My brother had a Mustang that just would NOT perform worth a DARN! It started ok and around town it ran ok it just very little performance. He fought with it for many months before he dropped the exhaust system. It was LOUD as all get out, but the beast really ran strong!
He found the muffler had failed internally, restricting the flow!
Who'd a thunk it!?
Sherm & Terry w/rescue Eydie (min Schnauzer) & Charley (std Poodle)
2015 Jay Flight 27RLS, GY Endurance (E), Days: 102 '15, 90 '16, 80 '17, 161 '18
2006 Ford F350 PSD, 4WD, CC, LB, SRW, Cmpr. pkg, Equal-i-zer 4-p, Continental ContiTrac TR