Hello, I am the original author of this game. I made it a while back (before SDL 2 was stable) for a game jam, so I'm happy to see this port.
One of the problems I see here is that only some of the assets are scaled down to fit 320x240. The blocks are too big, so there's not a lot of room (especially vertically) in the playing field. I'll work on making it easier to switch between half-scale and full-scale graphics. I should be able to fix the high-score screen rendering as a result.