What does the dealership mean by the oil leaking due to a warped head? Is the engine burning oil? A more common issue with a warped head is a blown head gasket which will cause coolant and/or oil to burn.
If the head is warped, you will need to have it surfaced and the block checked, as, it being aluminum, it may be warped too and need to be decked. The machine work for the head wouldn't be a whole lot (maybe $150) plus a head gasket and the work it would take to r&r the head (this of course would only be a reasonable approach if there are no other issues: e.g. piston slapping, rod knock, bent valve, excessive leak-by). If the oil is leaking onto the outside from the top of the head, it might be nothing more than the cam cover gasket.
You could smell the oil for overheating.
As far as used engines go, you can find them at a local salvage yard, ebay, or find a wrecked bike in the local classifieds that still has a running engine you can go and listen to/check over. Buying an engine used when you cannot see it run in person is a gamble; but I bought my TT600 engine off ebay and have since put 3k miles on it with no issues whatsoever. I suggest that if you do find an online salvage company, ask a lot of questions about the engine, ask for proof of mileage (a picture of the odometer, although it could be fake), and make sure they have a money back warranty with good feedback from other buyers.