Trouble starting

[Dave33]

I have a 2004 nissan sentra 1.8 230xxx miles. Put a 2005 1.8 engine in it at 200,000 miles. This engine had 43,000 miles on it when put in. Had to replace head gasket, a few months ago and had complete cooling system replaced. Car has had trouble starting since replaced battery maybe 1.5 years ago. Recently had check engine light diagnosed came up with crankshaft sensor and catalytic converter which has been bad for years. The sensor was making the car stall at low speeds and I ended up killing the starter both things have been replaced. Now the car is still having trouble starting. The spark plugs are getting some oil on the bottom of them, the shop mentioned the lower part of the engine is likely shot. But said I can pull the plugs and replace them with clean plugs and car will start. This seems to be the case sometimes but other times the car has a lot of trouble starting, it will crank and will sometimes start as I pump gas pedal, but it is a shaky start, and willbackfire sometimes. After started it runs fine. Any help or suggestions would be greatly appreciated. Or should I just try finding a new motor? A new car is not an option right now unfortunately.