O2 sensor and oil pressure sensor?

Alright so I recently bought the truck listed above, when I bought it, I noticed the oil pressure gauge was a at zero I asked the seller about it, he said it was just an oil pressure sensor and that it was a cheap and easy fix. Test drove the truck, and everything was running just fine, after driving it for a week the engine started to shake out of nowhere and check engine light was flashing. So, I pulled over and turned it off, but every time I would turn it on the engine would shake out of control, I was 10 minutes away from home, so I decided to keep driving it like that. The truck was shifting hard and very low on power (acceleration). The next day had someone come tow the truck to my mechanic, mechanic said he replaced every sensor on the truck and test drove it he said it was fine but then when he returned to the shop from test driving it, the oil pressure gauge went to zero and all the lights it had on came back on, and now he is saying I might need an engine replacement, I’m wondering if that’s necessary seems like the truck had a bad misfire and bad oil pressure sensor and 02 sensor, but does it really need a whole engine replacement for that? Please help.