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Sorry, you're way off there. My ancient car has a gigantic 90 litre fuel tank. At 720g / litre, that's 64.8kg. And that really is a huge tank - most cars are more like 40-60 litres.

The engine, meanwhile, well it depends what you count as "the engine". If we take it to mean "everything in the engine bay" - radiators, filters, pumps, etc. - you're going to be looking at several times that. Hell, even a battery is like 20kg.

What about everything else around the engine? You wouldn’t just move the engine to the rear but keep the transmission and the alternator and etc in the front.

Engines are only ~100kg usually

220lbs? Only the lightest ones. Let's also not forget the transmission/transaxle too.

https://www.gomog.com/allmorgan/engineweights.html

The fuel tank is partially empty most of the time when we drive, and many drivers never refill to the top of the tank, or don't put more fuel until the tank is almost empty. Those 100Kg could be on average 30 or 40Kg.