I've never done custom work on a gun or had custom work done; however, I found that AOA ran a YouTube vid
showing their custom gun shop.
Other custom builders exist, but since I have no experience with any of them, the video sticks out.
Google search showed Mac One Airgun Shop, Ronnie at Eason Airguns, and others. If one is close, you could do a bit of research on their reputation and see how it goes.
I'm finding that tuning the Marauder is very simple though. All you need is a chronograph, set of hex wrenches, compressed air, pellets and some time. Good weather doesn't hurt, and I've been lacking that lately. I'm experimenting, trying to find the most efficient tune for JSB 18.3 grain pellets. That's half the fun of tuning, I get to tinker.
I've got the factory settings, and Crosman will provide them to anyone upon request. If you mess up the settings or just don't like it, put it back to factory and start over. Takes about 30 seconds to get the three screws back. Highly recommend you get in and completely mess up the settings so you can get to know the gun.
As for parts like regulators and such, I understand the hesitation. Do consider though, parts like regulators are adjustable and if you don't like how it works, you will need to send the gun back, or learn to take it apart anyway. The costs of a custom shop may also be high enough to buy another gun along the way, so it's all that balancing act, what's your time worth. Custom shop might be the perfect solution for you where for me, I like to tinker.
Good luck