new to neon with 4 trouble codes

hey all, thanks in advance for any help you may be able to give me.

i recently got an 01 neon from my brother and started fixing it up again.

just had the starter replaced and a new muffler and flex pipe put on. car runs/sounds great.

he had put on a magnaflow exhaust but since has had it removed, i bought a new muffler from ebay and had a shop put it on for me. he also has an msd ignition coil on the car and some other things i'm not fully aware of yet.

the problems i'm having are with a check engine light and 4 touble codes.
p0442- small evap
p0455- gross evap.
p0833 - clutch pedal switch b circuit.
p1684- battery disconnected within last 50 starts (also found a site that said oil pump position circuit malfuntion?)
there is also a gas smell from the exhaust, i'm thinking o2 sensor? but not sure.

so now i have an 01 neon that need to get fixed so i can get it inspected and on the road.
anyone have any experience with these codes and or how to fix these problems? my brother told me the check engine light will always come on because of the exhaust as that is what has happened with him, but i have no clue. i'm fairly good with a wrench but by no means a mechanic.


any help at all would be greatly appreciated.
thanks again

Rob.