The subway would be useless, because the three islands are separate levels, and a map similar to GTA: Advance's would work, and the missions are not really GTA3 based, hence it's loosely based on the islands. Some problems may be from the geography of Liberty, but the Porter Tunnel would be rendered out,
Here's how it would go for the GTA3 loosely based conversion to GTA2: Downtown Level > Portland; Residential Map > Staunton Island; and the Industrial Area > Shoreside Vale. And it takes place in the future, around the same time as GTA2.
Why I brought this topic up? Since I played GTA2, I always wished there would be a map/mission mod that brought GTA3's Liberty City to a top-down game, and the dream wasn't fulfilled in GTA:Advance.