Three ideas:
1. Write a list of things that you need to do each really small implementations and keep it in front of you (I use google calendar tasks and keep it in an open tab next to you). Each time you finish a task highlight it or check it off. After a while, each unfinished bullet point will start to bug you especially the longer its been unfinished (at least for me). This helped me finish tasks I hate just because I hate it being the only unfinished task on my list.
2. I can't remember where, but I once read a statement by a coder who said that if a change is too hard implement [in code] restructure the code so that the change is easier to do. I would suggest that whenever you need to work with a section of code that you rewrite it so that you understand it better and can work with it. After all, its your project now, not the old developers. This will take time, but if you plan to stay with the codebase for a while, it might be the better option.
3. And then of course, you could just chug away at the project and work your way through it. If you just want to finish it hard and fast, I would suggest the Pomodoro Technique. I always prefer #1 than the pomodoro, but I've read a lot of people that swear by it, so it probably has some value.