I have a 2000 323i bmw with the stock engine it comes with ( 153k miles on it) and I def want to make my car way more powerful and faster. I had the idea to put a m3 engine in it but i seen on lots of forums there’s other engines that I can throw in that would be 10x better and easier. I also have a transmission problem , like a dumbass I turned my car off while I had it on reverse and my foot was on the breaks. When I tried to turn my car on a gear with ! Came up and I’m assuming I fucked my transmission up. I sometimes feel the gears slip and there’s a little jerk in the car when I accelerate. Is this a fixable thing or do I need a new transmission and if I do I want to know if there’s a better transmission I can throw into my car. It’s a automatic 5 speed , please let me know!