I'm glad you are trying to design a mass market car rather than a niche one, it has more impact to more people. If you are looking for a compact and cheap engine then the one in my car may be an option.
3cyl 1.0 and weighs 68 kilos. I have read of one guy using it to replace the 4 cyl alloy unit in his rear engined Hillman Imp because of its lower weight. It was also international engine of the year in the sub 1litre category taking over from the Honda Insight IMA unit in 2007 and beaten this year by the FIAT TwinAir - which is not all that impressive according to real world tests.
The big plastic box on top is the air filter, the engine itself is underneath. The nose is also quite short as shown here
and it could even be a little shorter if the tiny radiator is moved to the side.