I personally think that due to the quite major differences in SRB2 and Generations, you'd be better of making an entire new model inspired by Greenflower Zone Acts 1 and 2 from scratch that makes use of Generations' gimmicks and such, but if you want to make a port, that's okay too (though I have to agree with others here, you probably want to go with Sketchup as opposed to ripping, Doom's level format is FAR beyond weird...).