This is not really the right forum. The Modifications section is for game modifications that are released.
That being said. A cheap easy one I like to do is replace the plastic thumbstick with a PS4 rubberized one. It is literally almost a direct drop in replacement. Only difference is that you need to SLOWLY file down 2mm off of the plastic peg the thumbstick slides onto. After that, you will have far more grip in game.
Not sure how much the overclock mod costs as I was gifted my current Dreamcast by a close friend. But an overclock mod is great for certain titles. Even some games you may thin k would not have any slwodown do. Test Drive 6 is one game where the overclock takes it from having minor slwodown at spots to being 100% full speed no slowdown. Soldier of Fortune has some slwodown on the 2nd stage with the train and helicopter, using the 20% overclock mod removes performance issues. Some emulators benefit from the higher clock speed as well.