1998 Volvo S70 Craigslist crook. Plez help.

Engine Performance problem
1998 Volvo S70 Automatic 130, 00 miles

I just had my Head gasket replace and now my check engine light is on which was not on before? I also have leaky oil spots in the back of the engine which I think is leaking out more when I drive b-cuz not much oil is leaking out when im parked. Also after I drive smoke comes out from the back if the engine for about 5 mins. I know there is a leak maybe a bolt is lose? I hired a man off craigslist to replace my head gasket who garented his work with a signed a written warrenty, he told me the check engine light would go off after 100 miles I now drove 100miles and still the engine light is on I now relize I was lied too.
I try call him lots of times with messages, he gave me his address which matched his I.D. But he lives 2 hours away.
i just would like to get some advice im 20 yrs old and this is my first car I know very little about cars. MY QUSTION IS : as for the time being until I could have these oil leaks sealed, is my car okay to drive, I know I need to check my oil everyday but what about the check engine light do you know why it could be on. Also when im driving and I get past 40mph I hear a humming sound I dont know what it is. I know I have transmission fluid. Any advice would help. THank you!