Stick a fork in the car or pay to get it fixed.

There has to be more to the mechanic's bill than fixing a leaky timing belt cover. If that is all he/she did. they got you good. To address your borderline overheating engine, you should assess if water pump is providing good coolant flow, and whether or not your head gaskets are in good shape, but first look at the basic stuff - like coolant fans, hoses, thermostat, and radiator cap (if those are okay look at the harder stuff like head gaskets and water pump). You can buy or rent a product called "Block Tester" that will indicate if hydrocarbons are present in the coolant (these are a byproduct of combustion, which a blown head gasket would allow to seep into the water jacket). Another test for head gaskets is a compression test, again you can buy or rent (some stores even loan tools) a compression tester. If you can unscrew a spark plug, then you can run a compression test. Just follow the instructions and determine if one or more compression chambers readings are below the others. If it is, then the gaskets blown and likely the cause for overheating.

Radiator coils clogged with dirt and bugs causing little to no air flow across the coils clean radiator coils with garden hose and nozzle.