Just keep up the hard work, take your time when you need a break, and you can do it!
You've accomplished A LOT within a year. Keep in mind the size of the creative team and support staff at Stern, and what you and your family have accomplished.
It seems like most homebrew projects tend to fizzle out during the coding phase. Or someone does an amazing job with a layout and has some good code ideas, then phones in the artwork. It's for you so take your time and make it the way you like.
Besides, hasn't it really been about all the fun you've had with your nephew? Having a pinball machine is just a bonus at that point.