Re: The fin on the bottom of the motor
The "fin" on the very bottom of the motor, under the propeller, is called the "skeg". Its primary purpose is to keep the propeller from getting damaged when you inevitably hit an underwater obstruction. If it's missing a chunk it's no big deal. If it's bent a little it's no big deal. If it's broken off so it no longer sticks down below the propeller then it's a big deal. If that's the case then you should look into a thing called a "skeg saver" that goes over the broken stump of your skeg and makes it complete again. You do need the skeg to protect the propeller.
There are a huge number of "stabilizers" on the market. The only reason you would want to add any kind of stabilizer is to address a specific problem in the way your boat handles. If you're having trouble getting up on plane, or the boat shimmies at high speeds, that kind of thing. Go to the "boat setup" forum and do some reading. Stabilizers can work miricles or they can screw up a good thing. It's not like you can just bolt one on and make the boat go lots faster - it's not like adding a huge air foiler to a Dodge Neon!
