Engines can be found in any lootable area. An engine is needed with a gearbox in a vehicle to be able to use it.
Here are all the engines:
Powerful Goat/Roach Engine : (Legendary Engine)
+1100 Engine Power
+200 weight
+40 Top Speed
+400 Fuel Consumption
Standard Goat/Roach Engine : (Rare Engine)
+900 Engine Power
+100 weight
+25 Top Speed
+300 Fuel Consumption
Weak Goat/Roach Engine : (Uncommon Engine)
+800 Engine Power
+80 Weight
+250 Fuel Consumption
Damaged Goat/Roach Engine : (Uncommon Engine)
+200 Engine Power
+100 weight
+600 Fuel Consumption