Bad knock sensor, also half moon seal is dry rotted.

The left knock sensor has gone bad in my 2002 V6 prerunner. The check engine light comes on and off at intervals about every two days. Any idea what the cost would be with parts and labor? Is it a do it yourself job? Would this affect the performance of my truck if I just left it alone..??? Should have the new engine I recently put in it (due to previous owner) had come with new sensors?

Yet another problem is my half moon seals have dry rotted and are now leaking oil on my exhuast causing an awful smell. I had replaced the engine recently, but should have the new engine come wtih new seals?? are they easy to change out???

Any information would help me out a great deal!!
Thanks once again.