I looked at your save, and even played a little. With a few alterations to your gameplay, I think you will find regent too easy.
First, the things that need to be improved. City placement is not the greatest. You left a lot of unworked tiles in your empire, like south of Shimonoseki, I think it was, where you could plop an extra city. You have cities that are separated from the coast by one tile, meaning they have coastal tiles in their radius, but they cannot build a harbor or commercial dock to make those squares useful.
A lot of tiles are not completely improved, I would imagine due to a shortage of workers. You had six native and a dozen or so slaves that I noticed. At the end of the IA, everything should be completely improved and railroaded, but you were nowhere close. Railroads not only eliminate movement cost, but also make the tile more productive, producing an extra food or extra shield, depending upon whether it was irrigated or mined. Railroads are one of the most important things in the game.
You are also paying huge amounts in unit costs. The general solution there is to capture a few cities and in the process lose some units. That way, you can support a few more units, have a few less, and add to your economy and territory with a new city. Korea has only riflemen. With artillery and cavalry, you can take them out pretty easily (I did a few). You can also wait another tech for Motorized Transportation and use tanks, which could be even more fun, but you aren't researching.
This brings me to positives. When I opened the save, I checked to see if you were getting a bunch of gpt from other civs. When I saw you weren't, I was amazed at what you were making per turn with such a small empire. I found that you did a good job in specialization. You built Smith's, and having done so took full advantage. I didn't check every city, but I saw only markets, banks, and stock exchanges, but no libraries or universities. So lots of free buildings, making your economy a powerhouse. A strange way to go, when playing a scientific civ, but still a job well done.
You also seem to not be wonder addicted, as you have no AA wonders. Perhaps you missed them all, but I'll give you the benefit of the doubt and assume that did not try for any.
I was also impressed with the number of artillery you have. While I would like to have more, the number you had can do a lot of damage (and did, to the Koreans, even if it did bring the sniveling Japanese into war against us). As that was the case, use it!
A couple questions. Have you chosen a victory type? That helps in giving advice. And have you ever considered a Succession Game? It is tough to give tips for the whole game in one small post, but if you were to join a Monarch or even Emperor level SG and say you are looking to improve your game, you will get help in little doses throughout the game, which is much better than one big one. Or, if you have to play on your own schedule, playing GOTM can help too. Play the game, read the spoilers, see how others succeeded, and ultimately improve your game.
Hope I was able to help.
