When my friend and I would play RCT together, we'd pick a scenario that didn't have a time limit and just race to beat the goal. The goals could be certain number of guests, finish 5 coasters, build 10 coasters, etc.
As for a rubric, a very difficult part would be the park value. It diminishes over time and is directly affected by how much you put into it. Although it begs the question, is this competition just about the raw data and values of the game? For me, building a part is hugely a creative process that is mostly graded on its aesthetic and sometimes subjective criteria.