2002 ford focus missing badly,how can you tell if the timing belt has jumped.Front oil seal leaking badly.?

You remove the plastic timing belt cover and set the motor on #1 TDC. The cam gear timing mark then needs to line up with the mark on the back cover.

To fix a bad misfire you start with replacing spark plugs and plug wires. Then if it's still misfiring you swap the injector on the misfiring cylinder with another injector to see if the misfire follows the injector. if it does change the misfire to a different cylinder you replace the bad injector. When oil seals leak they need replacing.

.

2002 Focus. Probably has "vintage" spark plugs in it.

You should keep them and sell them to a hipster.

For realsies:

Fuel filter

Spark plugs

Check for corrosion at the coil pack terminals (the other end of the spark plug wires)

Make sure your plug wires aren't draped across shiny metal or exhaust manifolds.

Use a small spray bottle to spritz water over your plug wires and look/listen for sparks while it's running.

I don't remember where the crank sensor is on those things, but the oil "could" be contaminating it.

Doubtful.

Before making a diagnosis, more information is required. Petrol or diesel engine? Diesel doesn't have a timing belt, it has a chain. They are prone to injector issues though.

Get the OBD codes read to make sure it is not something else.