If you were anywhere near me I would.
Half the stuff on a modded car ends up custom fabbed...
All you'd need then would be a dyno & tuner and you could be a full performance shop that also manufactures & sells parts. I'm a DIY guy with anything that can just be installed but I've no doubt people would take their car to you to get you to install a bunch of YOUR parts, and anyone like me would take their car to you to fab anything they can't just install themselves so...
It actually seems like an awful lot of business you're missing out on.