Can I change an under mount sink for a drop in?
Yes an undermount sink can easily be converted to a drop in sink.  If you can find one that is close to spec all that you have to do is drop it in and secure with silicone.  You will have a little wiggle room given that you have a lip but it might be harder than you think - as undermounts have few standard sizes/look and these are not the exact same as top mounted.

To make your life easier if you can't find a good 1-for-1 replacement with exact aligning you are probably better getting something a bit bigger.  That way you can cut counter to meet the exact needs.  The other factors are any holes for plumbing.  You cannot buy a sink that that just covers up half a hole.  I would need to cover the whole hole or be like for like.