The regular APIs are designed to return the data ordered descending. For a performant solution one would need to implement own API calls on blockchain/hivemind level and provide own nodes which run the new logic or build an own database that is optimized for the new queries.
It's not just about forking condenser and changing a few variables. Most changes would have to be made on the backend.
Ah, so, maybe not as easy as, https://usesteem.com/steemwhales/ (which appears to be down at the moment).
Here we go, https://steemitboard.com/ranking/?p=54401&s=reputation