I think I got screwed out of a engine and need help finding out how to figure out engine vin?

We have a 2003 Nissan Altima 2.5, and while we were on vacation somehow the oil sending unit went out. So while driving home I had to stop every so many and check the oil, and I was fixing to pull over and I got to the bottom of the exit ramp and oil light came on, so I proceeded to gas station and whil pulling in it jerk a little. I checked the oil and it was out I filled it back up and it drove another 100 miles home, acted funny but made it. So my wife's uncle is a "mechanic" and he said we needed an engine so we replaced. Six months later it died on side of the road, I jumped it off and it started right up. But wouldn't stay charged so I had the alternator checked it was bad so replaced that and he hooked it up to the computer and said my crank position sensor was bad so I charged the battery and drove another 10 miles to my dads garage to change sensor and alternator. I changed the alternator and while changing the sensor 2 of the 3 wires coming from the wiring harness which goes to plug that hooks to the sensor, was ripped (I might had done it taking it off) so I got that plug and piece of wiring harness off the exact model car from junkyard. I wired it all back up and engine would not start. So I gave up and took it to the uncle again and he says we got the motor to hot somehow and we need another motor. I just don't see how the motor is messed up cause when I drove it in the garage the motor was running fine. So I guess my question is first off if in fact the motor is bad maybe he didn't change it the first time and I'm trying to fig out how the get the stick engine number to check the block to see if it wasn't even changed out the first time. And at the same time I still don't she how the motor is gone if I drove it in the garage and just changed the alternator and messed with that sensor. I just am a bad situation and don't know what to do. Would really appreciate any help thanks.