I can see a personal project in the shop at both ends of the spectrum. And if you look at shops you know you WILL see these two extreme ends.
1. The shop owner puts everything he has, time and money into the personal project. A race car is the worse. My dad use to say "Show me a repair shop with a race car and I will show you a shop on a road to going out of business". Neglect everything for the business because the personal project is more important.
2. The shop owner puts the personal project aside because he thinks he needs to work 18 hours a day to make money or he will go out of business.
I know I fell into the #2 category and it was all do to poor business practices and I could have worked 24/7 and STILL had to close the shop.
The shop owner should be able to work hard and make just like any other job, then have fun and work on that car when all the work for that day is done. Do as I say not as I do, as I didn't have an understanding of this simple concept.