Well, first of all, take it to any auto parts store, and have the codes checked. It may or may not be an O2 code. Assuming that it is, your mechanic pulled one of the most common technician mistakes, that is replacing the parts, without first finding out why they failed. This is the automotive...
of course you can! You can do what's called "plus sizing". This is a mathematical equation where you increase your rim size, while decreasing your tire sidewall size, so there is little to no over all difference in wheel diameter. Go here for a plus sizing calculator...
not sure how long it will take. But if the head is warped, you should have it resurfaced, or replaced. No amount of tightening of the head bolts will straiten it back out. Also, the gasket will need to be replaced no matter what.