Actually I think the re planting crops one (which is not a thing anymore) is the most logical way to get rep.
In case you didn't you Gabriel rep is short for reputation. Though donating to the server is good because it helps it out, it doesn't exactly mean that you are a good person on the server.
Winning the fishing contest to get reputation!?! In this server reputation is kind of the marker to tell how much you follow/don't follow the law. Gaining rep from a fishing contest would be unrealistic.
Now here we go... BUYING REP?!?! This devalues the whole point of reputation and will honestly change the server. Though I can see the server making some profit for a short time, this would be bad in the future. This would also have some other things to be changed like joining the police force. You said it yourself, Ciel looks at rep when selecting new police. If someone just bought rep then becoming a police wouldn't be any fun.
For the plants however.... (if you could) taking the time to replant crops for the community would make sense as it's kind of the opposite of braking the law. Since crops are auto replanted in the farm this would have to be re thought of.
In short, this is a needed idea that many players have suggested, but I think you need to do some more research on why. Gl