What is wrong with my 1999 isuzu trooper?

Just today I replaced my catalytic converter and the alternator because of rpm, stalling,not wanting to "go" and white smoke issues. It took the mechanic 3 days to come to the conclusion that this was the reason for it. As soon as I got my truck back the check oil light was on and it has a knocking sound, neither of these were present before. The mechanic is now saying that it is a motor issue. The same issues are present as before but with the others. What could the problem be?