Gen 2 to gen 1 engine swap

Is it possible to swap a gen 2 sohc neon motor into a 98 highline without heavy modification?

Yes you can, but you should stick to the 00-01 motors if you can. The 2002 and up get a bit more tricky.

So what modifications, if any would I need to do to put a 00-01 engine in my 98?

You want to use just the block and head. You would use your current manifolds, starter, PCM and so on. You might need to swap some of the 98 sensors onto the 00-01 engine.