My car engine was burnt up and i have been fixing it up evanescence. the last problem that i have now is, its using a lot of gas and the gas smells strongly when driving. i have changed the plugs and

The black sludge on bottom of engine is burnt oil. Scrape it off and see if it builds up again. Could have a leaking oil pan gasket. I would change oil and have a engine cleaner used to remove engine sludge. I believe engine runs rough when cold is because of sludge buildup and when it gets warm the oil moves more freely

have codes checked most auto stores offer this service for free and code will help in fixing problem --check your transmission fluid for level and look at color should be a burgundy color and not brownish or smell burnt also possible needs a filter change

check for oil leaks around the engine manifold check rocker cover for leaks if its a diesel check the turbo charger for leaks check wiring around exhaust manifold

You lost the drive gear in transmission. Check fluid for a burnt smell or a grayish tint this would indicate a burnt gear pack

Use an OBD II testing equipment to search for trouble codes. The burning smell can be from lubricant oil or a burnt out gasket.

Jennifer what type of timing belt does your engine use ?
Is it Nylon ? When was the last time you had it changed ?
Burning plastic smell may be being caused by the timing belt bearing runners seizing and the belt is getting hot .
If the belt is due to snap you could cause a lot of damage to the engine if it breaks.
Might be worth getting a mechanics advice on this as a possible cause

burnt or unburnt gas? if burnt gas smell look for bad manifold or manifold to pipe (donut) gasket.
if unburnt smell look for gas line leaks

When the engine is cool, check the fluid level in the radiator. If low add 50/50 solution of water and antifreeze. Do not continue to drive your car until this is fixed or you could cause a lot of damage to the engine.

it may b something as small as a clogged fuel filter or something bigger

Garyq1970 is right - white smoke usually indicates burning coolant. How full is your radiator? You should see coolant right up at the top of the fins inside when you remove the cap. Take a look and see if a) you are low, and b) if the coolant looks sludgy or contaminated, brown in color. If it's low, top it off and monitor it over a week's driving and see if you have to refill it again. If it's contaminated and brown, you have a problem.

Usually when you're burning off coolant, it's because of a torn head gasket allowing coolant into the cylinders. If it's not contaminated, that means the tear is between a water galley and the cylinder. If it's browish looking, then it's a tear between a water galley, an oil galley, and the cylinder, and the coolant and oil are mixing as well as being burned off. Either way, it's a very involved repair, as the top of the engine has to be removed to fix the problem. This involves removal of the upper and lower intake manifolds, and the entire timing belt system (which, given the in-depth job that the timing belt is, you might as well replace it, its tensioner, and the water pump and thermostat as well, to save yourself the labor later on). It's not a job for the faint of heart or wallet.

You may not be looking at such a repair but if you can monitor the coolant for a week or so and reply afterward and let me know, I'll take it along from there (just send me a request on here).